So this one's more mockumentary than found footage, but Noroi: The Curse (2005) is an absolute must-watch. It's a very slow burner, but when its secrets gradually start to unravel themselves, the previously-confusing plot becomes clear, and it's extremely creepy.

The Poughkeepsie Tapes (2007) - it's a mockumentary rather than strictly found footage, but it's one of the darkest films of its nature that I've ever seen, if not the darkest. It documents a serial killer investigation in New York, and includes footage filmed by the killer himself. The gold standard, imo.

Lake Mungo (2008) - mockumentary with a very love/hate relationship on Reddit. Some love it and think it's terrifying, some hate it and find it boring. I'm somewhere in the middle. A girl dies in a lake, and strange occurrences begin.

The Tunnel (2011) - Aussie flick that's gradually starting to get the love it deserves. A documentary crew investigates a government cover-up and delve into the abandoned train tunnels below Sydney, and they get more then they bargained for.

V/H/S - found footage anthology film (its sequel, V/H/S 2 (2013) is even better) that covers varying themes. The segments vary in quality, but it's well worth a watch.

The Dyatlov Pass Incident (2013), A.K.A. Devil's Pass. Personally didn't think much of it, but it's a decent enough flick. As usual, a documentary crew head over to the Dyatlov Pass to make a documentary about the real-life Dyatlov Pass Incident.

One I will always recommend is As Above, So Below (2014). One of my favourite films ever! Perfect for anyone who loves Dante's Divine Comedy (specifically Inferno). It's set (and actually filmed) in the Paris Catacombs.

The Taking Of Deborah Logan (2014) blurs the distinction between found footage and mockumentary, and it's superb. Really creepy.

Creep (2014) and Creep 2 (2017) - Mark Duplass is a mumblegore genius. I won't say much about either, because they're best watched totally blind, but they're both effectively about videographers hired by a man to record some videos for him.

Hell House LLC (2015). I was really impressed by it. I enjoy Halloween haunted house tour films (see: Haunt (2019), and this one combined HHHT films with the found footage genre. For a low budget film, it's awesome.

They're Watching (2016) is one that's barely talked about, which is a shame, because it's really great fun. An American home improvement TV team head over to Eastern Europe to meet an American emigrant and see her renovation. Only, the villagers are extremely religious and superstitious, and strange things beginning happen.

Gonjiam: Haunted Asylum (2018) - legitimately terrifying. As far as horror goes, this was the one that had me looking over my shoulder the most. A documentary crew sets off to an abandoned asylum for a live broadcast, and things inevitably go wrong. There are some massive jump scares, but they're not cheap ones.

The reason you haven't seen that many is because contrary to OP's assertion, there actually aren't that many found footage films. According to IMDB, in 2021 there were 1,388 horror movies released and of those only three were found footage films. If you consider every horror movie that's ever been released, or better yet every film that's ever been released, you're talking about a film style that makes up a fraction of a fraction of a percent.