For me its a combination of two factors. Firstly, I only play PbP games. This implies that at any given time you've don't have to be as prepared as over a table. You can make things up as you go along, starting only with rough ideas on a few pages. Secondly, I think it's laziness as well. I can't be bothered to read the vast amounts of material for published settings and get it right, that I'd rather just create something that I can't get wrong. It's just easier.
Enjoying the creative process, sure, but I think it's easier to create a quick and dirty world than it is to spend ages reading up on Greyhawk or FR, not even talking about all the supplements. Sure, you could set in in a reason, but I still think the entry level input it too high and too time consuming.
Pinotage
Edit: Cross-posted with die_kluge. :\
Enjoying the creative process, sure, but I think it's easier to create a quick and dirty world than it is to spend ages reading up on Greyhawk or FR, not even talking about all the supplements. Sure, you could set in in a reason, but I still think the entry level input it too high and too time consuming.
Pinotage
Edit: Cross-posted with die_kluge. :\