My guesses(!): Big topics are already crowded by big channels which get an algorithmic boost because of their past success. Its harder to get your slop prominently shown and establish your content slop farm with these topics. While with niche topics its easier to get your video to the top of the search results.

There are tools opensource tools like https://n8n.io/ to automate the creation of slop content. From what i heard its piss easy to setup. Like here is a half an hour tutorial to set you up:
https://www.youtube.com/watch?v=1u54cy2nQig

Then you shit out the slop content and hope your channel picks up. If it fails to drive numbers you abandon it and start with the next channel with the next topic variation. If you get lucky and the algorithm blesses you, you shit start to shit out hundreds of videos.

As seen eg in the video of redlyne about the dead internet theory. Where he looks a bit into the slop channels on youtube and other stuff:
https://www.youtube.com/watch?v=UnJKOYOkRMk