Why would you even want to search all the sites that are trying to sell you something when you're simply looking for some information? Putting it in pre-internet terms, it'd be like walking down the high street looking at shop windows to try to answer a question rather than going into a library to look it up in a book. Wikipedia isn't trying to sell you something - with the high street vs library analogy its more like a book than a shop.
> Why would you even want to search all the sites that are trying to sell you something when you're simply looking for some information?
Answering the question online is a means to an end. The 'answer' is the top of a marketing funnel that's designed to make you familiar with the answerer's product/service and sign up, or post a link to their article on your Twitter.
Look at it the other way around. If you were a cat owner, how much of your free time would you spend answering questions about their dietary restrictions?
Every other site on the net seems to sell you something.
Would you allow Wikipedia even though it tries to make you donate via those popup-like dialogs that slide in from the top?