Why no Wiki Books?

I hear a lot about how exciting it is that Wikipedia can be used as a textbook in developing countries. I have a better question: why can't it be used as a textbook in developed countries? If you are fortunate enough to be part of the population that owns textbooks I have a challenge for you. Look up a topic in your textbook, then look up the same topic in Wikipedia, I'll take even money that the Wikipedia entry is more clear. Granted this wouldn't work for all subjects, but it defiantly would for some. I think a big part of the problem is that professors have no real incentive to come up with an alternative to a text book...after all...they don't pay the bills.