Actually, they DO use intros and endings, breaks etc., it's just that they have to cue them up all by themselves, rather than having the machine take care of all that.

But I think a lot has to do with how the emphasis of the 'player' has changed from actually PLAYING the music, to 'directing' the loops and arps. As long as most of what you are doing is simply RH stuff (and not even that, some of the time), the rest of your attention CAN be directed to cuing up loops and triggering them at the appropriate time.

Not so easy if you are playing with both hands, though!