Something like 80% of reddit users don't interact at all and like 80% don't comment. So a tiny portion of reddit users actually generate comments.

Comments don't tend to be linear, more comments drive more people to engage which means more comments, so it forms a near exponential curve

Other websites HAVE seen and explosion in activity. Look at a similar graph for lemmy and you'll see a huge rise. Maybe not 1 to 1, but enough that if you extrapolate the same to other websites you can account for all the missing users.