Semi auto does have arc regulation. Its Ohms law. If the arc, for whatever reason gets longer (say the welder pulls away), since volts are related to arc length, this means volts are going up. Ohms law, if volts go up, amps go down, given resistance stays the same, though it really doesn't, but lets keep it simple because I have to.
Amps are related to burn off rate. So, as the arc gets longer, volts go up, amps go down, the burn off rate will reduce causing  the wire to extend beyond the contact tip farther because it ain't melting as fast, wire extends farther, arc is reduced, volts go down until the original voltage is restored.
Arc regulation.
If you really want to understand GMAW welding purchase a copy of Alexander Lesnewich's definitive work on the subject. Its one of the earliest WRC bulletins and is still, after decades, the best resource on the subject.

In MIG/MAG welding a constant voltage power suply is used. The arc length is determined by the voltage the machine is set at, and as the feeder feeds wire it melts off at a  distance from the work relative to the voltage. The position of the TORCH is controlled by the operator, and variations by the operator change the distance from the tip to the work, but as the wire is fed continuously, the actual arc length stays nearly constant. What varries is the ammount of wire sticking out of the tip to the point where it melts off. This is called "stickout".