Because they mostly generate Likes from bots, not from real people.
Do you care about Likes from bots? Does it add any value to your Instagram when a programmatic bot clicks your post? I doesn’t do anything for me. (Perhaps if you are a aspiring model or start-up business and you feel that you need good Instagram numbers, regardless of their source, you might be happy with “bot-likes.”)
I ran a little experiment, shown below. On a test Instagram account I posted 6 pictures, all with the same set of popular hashtags. Five of the pictures were reasonable streetsyle photos; the sixth was a plain dark square. The five reasonable pictures, on average, generated 36 Likes, while the ridiculous plain dark square, hashtagged the same as the other, garnered 29 Likes. Presumably no human being, except possibly an undergraduate majoring in Abstract Art, would deliberately Like a plain dark square. But the bots only know hashtags and don’t care anything about the quality of the image. So, presumably, bots generated 29 Likes on *all* six posts, and humans added 7 additional Likes on the legitimate photos. Oh my!
By the way, please do not Follow my test account.