Because they don't care if you can search. In fact, they probably don't want search too be tooo good.

* Good search means you could find stuff easily

* Bad search means you're more likely to post again and continue the re-engagement circle.

For a platform that drives revenue through continued engagement, clicks, views, and activity... could functioning search be considered an anti-feature?

If I can't find what I want, I'll start a new post, or reply to a comment… leading to people getting notifications, leading to clicks, more replies, …