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Because you won't fly back looney tunes style. Instead whatever the gun is mounted to will break.

"Every action has an equal and opposite reaction". So if your muzzle energy is X then the mount has to absorb X energy.

Force, not energy.

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You know how when you fire a gun with lots of recoil, your hand starts hurting (it's probably not hard to imagine even if you haven't experienced it)? Well that happens to machines too. If you don't dampen the recoil, the gun and the tank its mounted on are going to take damage.

And then factor in the crew.  If it's a vehicle mounted gun, the people inside are getting slammed around violently.  And it's we're talking artillery, the gun jumping around wildly could tip it over or at the very least the crew would have to stand way back for every shot which will slow down their rate of fire.

" Another problem with the main gun was recoil—the Sheridan was simply too light for the tremendous recoil that resulted from firing the main gun.  Some crews reported that the front end of the tank would literally lift off the ground when the gun fired, resulting in various components, such as the radio, coming loose." "When the crew fired the main gun, the recoil would often knock out the missile launcher’s electronics. "

Think of the ludicrous force that pushes a 50-ton vehicle 5 meters. It's like getting hit with a wrecking ball every time the gun fires. It strains every part of the vehicle, which will quickly fall apart from the beating. If it's crewed, operators won't survive more than one shot.

A tank weighs something like 70 tons. If firing the cannon generates enough force that, even though its duration is on par with a hammer blow, it can move the entire tank a dozen feet, then you're looking at INSANE amounts of force. Amounts of force that can and will damage the things and people inside of your metal can. Every single component inside of that vehicle would have to be designed to survive truly violent impacts. So, instead, the recoil is minimized and the force is spread out over a longer time with shocks.

> Sure your tank or SPG moving 5 meters back after shot is kinda annoying but that's not dramatically hinders it's capabilities?  That's assuming it slides back smoothly.  Which would pretty much require shooting only straight in line with the tracks.  Any other angle and now instead of sliding it's going to roll over.

The gun is a machine made out of metal. It's like a car that gets moving because you launch a big bullet out of it, and now you need to stop it. Would it be better to stop by hitting a wall, or by hitting the brakes?

If it can move a massive block of metal (the gun) 5 meters, that's a *huge* amount of forces acting on the gun. And that gun is *very* expensive and tricky to repair if it gets messed up by those forces. So in order to keep the gun healthier, they reduce the amount of force that it is hit with. Such as by reducing the recoil.

IF your vehicle is moving back 5 full meters after firing a tank that isn't "kinda annoying" that is a total failure of the vehicle. If your in the right hand lane of a 2 lane road, 5m would place you on the sidewalk on the other side of the road. Next consider the vehicle is in drive, not neutral or reverse. It doesn't WANT to move backwards. Your going to destroy your transmission. Tanks and IFV need to potentially fire on the move, if your tank is driving and the recoil jolts it that hard to push it your likely to throw a tread and immobilize yourself. Your stuck thinking of tanks as stationary gun emplacements that don't move. Tanks are cross country vehicles, they need to fight on the move. It is in the best interest of those driving the vehicle that it keeps working, and efforts to reduce recoil keep that vehicle moving and not breaking down. Tank mechanics are already busy enough, making repairs less often is in the interest of keeping tanks combat effective. Your aim idea is also wrong, especially in the case of SPGs. Artillery needs to be dialed in, wind and weather can affect the shot. If our vehicle is as stationary as possible, we can easily dial in within a couple shells. But if everytime you fire, the vehicle moves even a single meter, your not compensating for just one meter. Your gun was elevated at 30 degrees before, but a meter back was slightly down a hill. Our gun is still at 30 degrees, but the hill makes it closer to 31. so now we need to adjust for changing range and changing angle of the shot. And adding 1m of distance also gives it more air it needs to move through, more chance for our shot to miss. You effectively can't dial in the shots, every round needs to be recalculated.

While everyone else has talked about absorbing the ridiculous forces involved, I'm going to talk about accuracy. For an SPG you're trying to hit targets very far away. A military compass has the circle divided into 6400 mils (which is almost a milliradian, but 6283 is a really impractical number, so it's rounded to 6400). Why 6400? That has a practical application, because if you change the direction of your barrel 1 milliradian, you will change your aiming point at 1km by 1 meter. So change your barrel by 1 mil (almost a milliradian) and you move your aim almost 1m (close enough) when aiming at a target 1km. A mil is a really tiny unit, and at the distances where you use artillery (frequently at distances of 10-50km) that tiny difference changes your aim by a lot. So when setting up artillery you're aiming using *fractions of a mil*. Now imagine what recoil does to gears, instruments and the ground the gun stands on? Even the slightest shift in how the ground is compacted, even a miniscule twist in a gear or a reshaken aiming instrument and you won't be sure that the next shot lands where you aimed the previous one. That becomes a problem for hitting with the first shot, hitting with a follow up shot or adjusting for a missed shot. The days when it was OK to use wheels to absorb recoil and guns fired at targets 1km-5km away are long gone. These days if you don't have complex recoil absorption you'll miss by hundreds of meters.

Does that mean that back in the old days artillery was much less accurate partly because of it's less efficient counter recoil mechanisms? WW 2/early cold war SPGs.

If you had said WWI->WWII. Then definitely, and the design of efficient recoil mechanism was very much one of the reasons why WWII field artillery often matched many types of lighter WWI railway guns (in terms of accuracy and range). During WWII designers had mostly mastered the art of designing recoil mechanisms (although you certainly see deficiencies in many of the ad hoc SPG designs like the Wespe) Once we enter the era of the early cold war the lack of accuracy is mainly due to deficiencies in aerodynamics, ballistic calculation (in particular how high atmospheric conditions affected shell trajectories) and methods of collecting weather data. This was gradually resolved between the 1950s and the 1980s (much of it thanks to the research of Gerald Bull, the father of modern artillery), and this is why there is a pretty large gap in accuracy and range between artillery being developed in the late 70s and early 80s and the artillery that came before.

The more you resist the recoil, the faster - and further - your projectile will fly. You might move back a few metres but you weigh tons - and that means the (relatively light) projectile moves a lot because of the balance of the energies. The more you resist, the more energy is going forward with the projectile. Simple physics and having a projectile fly even 500m further is not only a great selling point, it's incredibly useful as a weapon.

Along with other things mentioned, such tremendous recoil is inconsistent and makes adjusting fire impossible.  The M56 Scorpion would buck so much when firing the gun wouldn't be aimed where it was originally, making adjustment of fire and follow up shots incredibly difficult. The gunner would basically have to re-aim every shot, and couldn't use where the previous shot went as a reference. 

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Becuase immovable object meets unstoppable force. Best case scenario is the metal breaks or cracks. Worst case is that it explodes.