VTChEwbecca
kickbox
You can't lay all the blame at the doorstep of the graphic designer. You've got the illustrators and photographers too. There are art directors, creative directors, executive creative directors, etc. Then you've got the publishing house themselves and any number of persons on that end.
I don't work in the publishing industry so I can't say for certain, but too many design agencies fall into the trap of design-by-committee. EVERYONE has an opinion, and some (though they would never admit to it) will push for assinine changes simply for the little ego boost they get from having put their personal touch on it. So few care that the designer is more knowledgeable about color theory, composition, typography, and design trends..
I agree that it is probably too many hands in the pot - the US covers often look over-done compared to their UK counterparts. My husband works in advertising and often sees clients muck up the designs of the graphic designers ("Can you make my logo bigger?") because they just don't get it. The big-wigs up in the main office and the sales reps can be almost as bad as the clients some days. Perhaps the UK book publishers allow the designer to do his/her job without allowing 20 other people to mess about with the design.