I doubt Reddit builds a decent search engine, that doesn’t actually help them at all.
If users can search, they find a previous post pertaining to what they want to see/know and they move on.
If there’s no search, users can’t find old posts or comments so they make new posts about a previously posted topic and more comments are made as other users react. That’s more content, even if low quality from a user perspective, that shows engagement which can be sold to advertisers.
That’s before considering the engineering effort it takes to make a good search engine, constantly fine tune that algorithm, and try to outpace those that are trying to game the search algorithm.