Because it takes time and money to make Content.

I'll stop you right there: I don't give a shit if they pirate every single game they play. It doesn't matter. Because, even amongst the streamers, you are looking at hours of prep per game (to dial in settings, weird streaming hiccups, etc) and on the VOD side it is generally accepted that you have hours of footage and editing for every minute of Content.

And all of that costs money. Being able to stay up late to write a script to make that Dark Souls run really cool? Doing insane after-effects editing to do a stupid joke star wipe? Or just playing the same cutscene over and over so that you can get the right background NPC for your gag. That takes time.

And you know what helps with time? Money. Which comes from revenue and "engagement".

And this is very demonstrable. Plenty of youtubers and streamers have very clear differences from their early work to their new work. A great example is Michael Reeves (who I assume is not cancelled just yet but...). His early videos are awesome. They also are incredibly low budget and often rushed. Whereas his newer videos (even the one where he just drives around in a sandstorm for a while...) have ridiculously good production values and involve some real feats of engineering. The difference? Before he was part time flunking out of school and tutoring for a living. Now? He... nobody is really sure how Michael Reeves makes money but I assume OTV pays him a good salary for showing up a few times a year?

Also: People vastly underestimate how much storage and bandwidth is required for video. Which is why peertube and the like basically exist for proof of concept one offs and for companies to fork and use in their own products.

Ideally it wouldn't be, but corporations will use whatever video platform is popular to pump out videos designed to increase engagement because to them it's advertising. They will try and sponsor their content on whichever content creator is on said platform with a large audience.