Vibrations. The less things the barrel is attached to, the lesser they are and thus the gun is more consistent and this leads to accuracy. That's why the good bolt-actions have bedded barrels and mostly the only thing the barrel touches is the receiver it's attached to. Also, when shooting the barrel warms and most metals expand when heated, if the barrel is free floating then it doesn't bend the barrel, thus doesn't move the point of impact. If the barrel is attached from 2 places and the lenght of barrel between these 2 mounts increases a little, the barrel suffices a slight bend, AFAIK.
Sure, there are several very precise semi-autos, where everything that is attached to barrel is fixed so that the vibrations are at least consistent ones, like 1/2 moa guns PSG1, Mk11 mod0, or 3/4 moa guns, like MSG90 and M21. But semi-autos (especially the mentioned ones) tend to be more expensive than bolt actions of same accuracy.
As said before, it's a thing of manufacturing standards and quality control.