its pretty easy for fortnite/pubg/etc to ban snipers, the people who do it constantly join a lobby and exit if they don't see it sync with the twitch stream. you catch someone jumping lobbies hundreds of times in an hour (without participating) and only stay when they catch their target and its a trivial catch/ban. Battle Royale games make it super easy to catch em, its much harder in something like Fallout 76 or MMOs etc but its also infinitely harder to stream snipe in those kinda games.
I think people like ninja also tend to have a stream delay of like 1-2 mins so its much harder to (consistently) connect to the lobbies they are playing in.
This is not a case of some dude happening to notice "whoah i'm in a BR game with ninja, lemme go hunt him down and kill him" - that happens - but there are thousands of people who actively try to get onto the same game ALL the time of their favorite streamer and those people are essentially digital stalkers and just as nuts as any hollywood ones
fortnite has definitely raised the levels of a lot of people. I followed nickmercs for years, he was a medium time Call of Duty streamer with 4-10k viewers and that dipped hard as COD became uncool, he was down to like 2-3k viewers for a few years but he's blown up hard since fortnite. which is crazy, he is definitely not a good FPS player - he plays with a controller on PC lol - but he is entertaining and funny in a "swole bro" frat boy way