Introducing ads on Wikipedia would likely do more than show you ads and have ad providers pay Wikipedia; it would also introduce third party tracking.

Adding ads to Wikipedia would give ad providers the ability to trace your Wikipedia activity. This would be horrible because the average person would not understand that their wikipedia history was being revealed to third parties and this history is often highly personal. For nontechnical users who do not know about this tracking, this would be deceitful. For nontechnical users who do know of the tracking but do not know how to stop this tracking, having ads would severely compromise their ability to use Wikipedia since they must then become cautious of the way they browse.