Two or three possibilities about the 'great talents':

1) They are no worse than a lot of other people, but no one bothers publicizing the unflattering stories about the other (unknown) people;

2) The great talents we hear about are/were also celebrities, and all the weird stuff that goes along with that brings out the worst in them (which doesn't explain obnoxious behaviour in their pre-celeb. days);

3) An inordinate number of great talents suffer from some form or other - or multiple forms - of mental illness, diagnosed or not. Or, there is something in their brain/mind that is not functioning 'normally', so that their way of seeing and responding to the world can seem to others bizarre if not outright obnoxious, at least when it is not producing art. So, for instance, someone with Asperberger's may seem rude when actually they have missed certain social cues that others pick up on.

4) The drive to create and/or succeed was a response to an unhealthy childhood environment which otherwise contributed to the formation of an unpleasant personality.

5) Various combinations of the above.