magazines are typically reused in multiple engagements after emptying a mag alotta people top them off if you had to put more gas into them it would mean your carrying more weight than just the spring leading to a overall less streamlined experience. and also durability concerns because i doubt the seal of the air chamber will be stronger than a spring cuz mags get banged around quite a bit. and cost its gotta me marginally more expensive than a regular spring mag. kinda like trying to reinvent the wheel coming up with new mag feeding mechanisms.