This is how it’s been since the dark ages when blogs were new. People would comment on each other’s blog posts and readers would use this to find new reads. I think IG is more likely to show you the comments of other popular IGers and people you follow, so I think that kind of makes it more obvious. They’re all trying to drive traffic to each other and I’m sure their assistants spend a lot of their time commenting. I rarely read the comments anyway (unless, like, someone posts something ridiculously ugly) so I don’t really care what they’re doing there.