Not condoning it, and maybe this is a bit paranoid, but if I was a big Twitch streamer and the majority of my chat was hateful assholes, I'd be a bit worried about calling people out seriously, thinking they'd group up and dox me or SWAT me for shits and giggles.
Plenty of Twitch streamers have moderators that keep the chat mostly clean and on topic. Just like here on GAF, if you curate discussion then you don't seem to have as many shitty people in your chat.
Generally, if a big streamer has a bunch of racist shit in their chat, then they're either too lazy to have it removed or they're tacitly approving of it.