The Mastodon argument against generic search is that it leads to pile-ons. If you want to opt-in a post to be searchable then you expose it via hashtags. To a Twitter user it feels a bit over-exuberant new social media manager, but once you understand the context it becomes more understandable.
Unfortunately not enough people use it. And realistically, in the middle of a formula 1 race or a ufc fight, there’s no time to be hashtagging each and every keyword if you are live sharing your thoughts or replying to others. A single key hashtag is fine like #AusGP. But when I want to search for Albon and Red Flags and opinions on that, I can’t expect people to do #Albon and #RedFlag. Full text search is sorely missed :,(
The real reason is mostly just speed. Pleroma has a full text search and its frankly kinda ass in terms of speed. You need fairly beefy hardware to make full-text searches an option if you're dealing with that many "documents". Doubly so on Mastodon, which is already fairly bogged down by being a Rails application.
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.
There's another aspect to Mastodon search that distinguishes it from Twitter. When you search you do so in your own history of prior interactions. It is like 'personal search' that way and becomes more valuable the longer you are active on the Fediverse.
