I agree mostly with mark.
I think partly it is down to workload.
Some of the big names are playing so much (DJ wise) that they may not have the time to be creative in temrs of personal exploration.
ie the little time they get in the studio, whatever they make in that time, needs to get released, crap or not.
And it also may be partly down to lack of inspiration. When you make the same music again and again and again, it`s not very inspiring, or good for evolution. So the creative passion wears a bit thin.
In some cases, I think just taking a break may help.