I agree. I find it interesting that we took search on Twitter for granted and only realize later that it is probably more important than expected. Maybe a search engine for Mastodon can fix this.
The infra can't really support it without heavily centralizing the model (something for which Mastodons questionable tech stack already doesn't do itself any favors).
There's also a bit of a privacy concern in that many instances just don't like it when big nameless entities start scraping the posts of their users, which also leads to a fairly hostile mentality towards the concept.