The thing is, for many communities public search is an anti-feature. This is obviously true if the community is private, but most public communities are centered around something that's monetizable. The incentives to create promotional content in established communities where there's search is much higher.

Volunteer mods then have to deal with people who's entire job is monetizing their community and eventually give up - causing the community to rot.

Search just isn't worth it for anyone actually making the content.