This is the real answer. I forget the exact numbers, but the vast majority of people on reddit are just lurkers. When you have an enormous user base, that still translates to lots of content to consume. Lemmy has way less content and very small communities (if any) for most niches.

Of course you can point to bots on reddit inflating those numbers and that Lemmy has more meaningful interaction, but that’s not what most are looking for that are on reddit.

Also, as others mentioned, there’s no negative engagement algorithm drivers on mastodon like there is on Twitter. Fact is, a lot of people just like to be angry and combative.