I think part of it is some seed drops just go quick and then they're gone, like for instance heres someone that sells a bunch of stenocereus seeds that I know quite a few people were looking for some months ago
https://www.etsy.com/shop/MigsTwigs?ref=usf_2020 granted they are overpriced but sometimes you just never know when you'll see them again
(also for the record I have never and did not buy from them, but I know someone was talking about them earlier)
or maybe there's a secret seed cabal that buys seeds and burns them to keep them rare 😂
but seriously though I think a big thing is language, for instance I was looking for some authentic taiwanese chili varieties that just weren't showing up, reached out to someone on the taiwan subreddit and they helped me find it, didn't show up in english but just fine in mandarin