The same reason why Apple is not in the search business even though search is a key part of the phone experience: someone else does the job really well.
Facebook would need a way to differentiate the search experience. I don't think personalization is enough since Google personalizes too.
I also don't think a company should build products for the _purpose_ of revenue. Revenue is the output of building a great product with a purpose for users. So I think it goes back to Facebook asking what unique purpose would it provide in it's search experience?
Facebook should get into the search business for the same reason that Apple is getting into the software subscription business, it's an obvious way to grow their business.
Facebook already has the analytics in place from their share buttons, ad platform and user engagement on the feed to create a great ranking system. A lot of people already use instagram to search for restaurants, fashion, travel and products, same goes for facebook marketplace and business pages, all they'd need to do is expand the search bars in their apps to include web results. Tracking search queries would give them true intent data that they could use to improve their ad targeting across their whole ad platform.
Facebook would need a way to differentiate the search experience. I don't think personalization is enough since Google personalizes too.
I also don't think a company should build products for the _purpose_ of revenue. Revenue is the output of building a great product with a purpose for users. So I think it goes back to Facebook asking what unique purpose would it provide in it's search experience?