The 2017 Vegas shootings were like that (which was full automatic in practice if not in legal or technical definition).
Most mass shootings, though, aren't like that. People aren't clumped up like they were in that case. Also, most people tend not to be careful about shooting in bursts, which helps control your aim. Even an AR15 (which has relatively mild recoil) will still walk all over the place if you hold the trigger down.
The NFA had organized crime shootouts in mind. In theory, the mob could do the kind of training as a group where full auto makes sense. Even if they would, that's not really the threat posed these days.