It's because they were told sometime in the last decade that you could make a site profitable through ads alone. Because it worked for Google so incredibly. They forgot that Google works because they target massively relevant ads to people who are looking for links to begin with.

Interestingly enough, Facebook's revenue plan involves making money off of Live Search ads, as of the last month or so. Essentially, they're ripping off Google as honestly as they can: by emulating them. I'm interested in seeing how they'll fight against embedded Google search: that's their huge competitor right now.