No ads Wikipedia?

I don't understand. Why can't Wikipedia post some ads on its pages to make up for their operational costs? If they integrate Google text ads it won't be a distraction for the readers while it may even provide some relevant commercial information to them. Since editing Wikipedia is made by volunteers, the revenue generated from these text ads should be more than sufficient to maintain its operation, given that Wikipedia is the fifth most visited website on the World Wide Web. It's not like because you are a non-profit organization then you cannot post any ads on your website; it's not like advertising is an unethical conduct - and if it is, then Wikipedia needs to remove that strikingly colorful photo of Jimmy Wales from its billions of pages.

Posting ads is good to Wikipedia (so that it does not need to go this far to beg for donations), good to its readers (so that they don't need to face Jimmy Wales every time they use Wikipedia and may also get some relevant information in the text ads), and good to the economy as a whole (after all most commercial activities will indeed increase the revenue of the whole society). Then what's been hindering Wikipedia?