They have revolting covers because ebooks typically sell a few tens or hundreds of copies, and occasionally a few thousand, in comparison to the tens of thousands of copies the typical romance or sf mass market paperback will sell. That means there isn't the money to pay for an artist's time to do the sort of cover you see on the mass market paperbacks -- see alg's post on profit and loss worksheets for an idea of how much money is spent on those at Tor:
http://alg.livejournal.com/84032.html

Not that the covers on the Big Publishers' books arer always wonderful, as the Smart Bitches like to point out.