Part of the problem is the sheer quantity needed (which also affects places like Harlequin, but they've got more money to throw at it). A place that's putting out two or three books a week gets through a lot of art, and often on fairly short notice.

None of which is to say that you can't get decent art on a budget. Loose Id does the occasional sporkable cover, but the catalogue as a whole looks fairly reasonable. However, they also realise that covers sell books, and are willing to spend time and money on art, within practical limits. I've been pretty happy with my covers:
http://www.loose-id.net/searchresult.aspx?CategoryID=30

There's one series that's pretty obviously Poser, but it's actually appropriate for those books.