sigh Are we having this discussion again? He pre-records bits and pieces of his tracks - the drum beat, the bass line, different variations of both, melody, samples, etc... and then puts them all together and manipulates them live at the show. It's how all DJ's perform - what do you expect him to do? Play every single note in real-time?

If you've ever used Ableton Live (I assume you haven't because you don't sound like you know what you're talking about), you know that it's a program built for this exact purpose - live performance of loop and sample-based music. He can set up the track to start however he wants, and program it to loop a certain amount of times, turn on and off clips (which is just a small snippet of a beat, line, whatever), and the knobs and faders and buttons he is playing with can control literally anything in the program, allowing him, who knows the program very well, to do whatever he feels like doing with the song.

Or would you rather watch Weezer play the same 9 songs, the same exact way, over and over again at every show? A solo DJ, like Skrillex, has the freedom to do whatever he can think of on stage. A band like Weezer or The Rolling Stones are constrained by what they rehearsed. If the bassist decides in the middle of a song that he wants to extend the bridge a little bit longer, well he'll just have to wait until after the show, bring it up to his bandmates, and hope they like the idea - and then remember to rehearse it the next day...

... totally exciting and spontaneous. /s