They do it because of the same reasons they do the brainrot activity?I used to watch a lot of streams about another videogame (shooter). It is calming, no hard thinking, sometimes the streamer reads your comments which is the dopamine moment, also there is an ability to listen to some kool kid without a risk to be bullied. In my case it was a learning about how to play shooter games, what is FPV/TPV, where the danger is coming from typically, how to play while being on a low ground, or a high ground, how to use different gun parts. Sometimes donaters use to send payed challenges for the streamer like win the game having some handicap for which is always give some extra bits of entertainments because I know the streamer has a hard time.
Last but not least is the reason that my computer is a bit slow so connecting to the game takes a lot of time and there is a risk to be killed early which means at least 10 minutes of concentrated mind wasted, but seeing the streamer killed means I am in one click from another game on the culmination point.
What is not understandable by me is why Minecraft is interesting at all. Watching a proper shooter game is mostly the same as watching some military expert telling (for example) about how to clear a building, but playing blocks stopped being interested for me as soon as I have been introduced to some real-life activity like soldering some electronics - but there were no brainrot in my childhood.