Yes, that's an interesting insight.

Depending on the other sites you've in mind, maybe it's because they have lots of content creators as well?

I mean that content creators (pic/video/whatever) interact between themselves a lot. But content consumers I would think much less in general. Same as people living a review after buying a product/experience.

You see what I mean?

(anyway, these are wild guesses. I would be curious to have some stats from Medium side on this. Like number of comment distribution per reader/article)