Pro Surfers Boards - Why so plain?

I've been watching alot of surfing on TV lately. X-Games, various videos, Huntigton Beach etc. and one thing I seem to notice is that most of the pro-boarders ride very plain looking (usually boring white/ no artwork with a few stickers on them). Do their sponsers make them ride a plain white board to show off a few sponser stickers (like a billboard). It just seems to me that a surf board is a great medium for artistic expression of an individual (like getting tatoos, designs on skateboards, snowboards etc) and most surfers you would think would like the option to have a great design on the board. I just don't see it on the top pros boards and was wondering why? Money obvoiusly is not a factor for them since they get the choice of the top technically designed boards.