Instagram's tagging system was and is atrocious in combination with their discovery mechanisms and the incentives they create.
A real example, this has been true for years: I want to look at pictures of Jennifer Lawrence's makeup because she, like me, has hooded eyes and that makes useful reference. I go to instagram imagining that I will find fan accounts posting pictures. I search for #jenniferlawrence. 2.8 million posts. Nice.
https://www.instagram.com/explore/tags/jenniferlawrence/
Only three of the top nine pictures present there today (same on mobile for me) are of Jennifer Lawrence.
This is what happens when all the engagement is counted in one big engagement bucket, all eyeballs are equal eyeballs, and all likes are equal likes. There are two gorgeous pictures of Anne Hathaway here, and I'm sure they've gotten great engagement, but they are absolute shit at being pictures of Jennifer Lawrence. So now I have to scroll through a ton of absolutely irrelevant nonsense – with attendant ads, let's not forget the ads – if I actually want to use this tag for its sole raison d'etre.
For contrast, consider what happens when a picture is cross-posted on Reddit. If I upvote it in one subreddit, I am giving an engagement signal specific to that subreddit – and I as one user may choose to downvote that same picture in another if I think it doesn't fit there!
This isn't limited to low-effort reposting of professional photographers' work. It happens in less egregious ways for many, many tags in many areas I've seen on IG for quite a few years now (though I can't speak for the whole history of the app). Artists tagging media they're not using, etc. etc.