In short; because all the range beyond 60ft is entirely the result of the magnus effect.
In a firearm, range is derived from muzzle energy, projectile weight, and projectile shape. The rifling only helps to prevent it from tumbling in flight.
So if you want more range, simply increase the weight and muzzle energy of the projectile.
In airsoft, we have a fixed muzzle energy, and a fixed projectile shape, and so we require a completely different principle of physics in order to get more range.
So we have the magnus effect (measured as backspin), and BB weight.
It's high speed projectile physics versus low speed projectile physics. Someone once asked me why they never applied hopup to a musket to produce better range and accuracy; well first off, you'd need one hell of a rubber patch to apply the right hop to a 3/4" or less steel ball, and secondly because the magnus effect only gains it's effect at lower forward velocities. A spheroid with a backspin will increase in magnus effect as it slows down, that's why we have straight flight paths with BBs; as the forward velocity slows, the magnus effect picks up, but the magnus effect is also slowing at a proportional rate. That being said, people generally aren't concerned with the performance of a firearm projectile after it's slowed below lethal velocities. Whereas in airsoft, we ONLY care about the sub-lethal performance of the projectile.
We can't rifle a backspinning BB, firstly because it's extremely difficult to apply a second axis of spin to an object that's already spinning very quickly in another direction (gyro effect). Secondly, because the magnus effect specifically requires the BB to be spinning up against the force of gravity, a second spin would force it to spin on a diagonal axis, causing the BB to consistently hook off sharply to one side (as if you were tilting your gun).
Now looking at actually implementing this, there are 2 ways to get the projectile to pick up that rifle spin; the first being simply having fins on the projectiles to have them pick up spin as the travel through the air (like paintball first-strike rounds), and the second would be to force spin by pressing the BB through rifles in the barrel, which would require a ton of force and likely result in wildly uncontrollable muzzle energy.
However, it's significantly more difficult to stabilize a non-spheroid projectile in a bore if you DON'T want it to touch the walls. So you might pick up way more barrel fouling with the first-strike type rounds, as the projectile would have to at least partially be in contact with the barrel wall in order to stay straight. So the accuracy you gain in having a rifled projectile may be lost very quickly as the barrel fouls.
Also, since the projectile now has a long cross section and (potentially) aerodynamic fins, it's going to be significantly more affected by the wind.
It can be done, and has actually been prototyped no less than 3 times with differently shaped projectiles. But in the end, it simply doesn't produce more than 60-80ft of range, the BBs would be very expensive, and it's ultimately just not as accurate as spheroid projectiles.
Last edited by ThunderCactus; September 8th, 2015 at 12:42..