> I wonder if there's a bit of game theory in play where it's impossible to be discovered when playing a megapopular game like Overwatch/Fortnite, and it's more advantageous to play relatively more obscure games.

I do not think most twitch streamers are trying to be professionals. I think most streamers are just trying to add more fun into their game time. In game theory everyone's main objective is the same, in this case measured in either revenue or viewers. In the twitch world most streamers are trying to maximize fun instead of viewers or revenue.

(Disclosure: I stream relatively obscure games, and know many other streamers in both megapopular and obscure games.)