While it is indeed true that humans share a large number of traits that naturally homogenize, to an extent, how we think and act, I'd bet it is ultimately Instagram's interconnectedness between users which favors this kind of content replication. Quantifying social validation by means of like, comment, and follower counts make other users' reactions predictable and, thus, exploitable; I think people who take this kind of hackneyed photos or videos are doing so because they have been proven to work--they already have feedback for that kind of aesthetic and narrative artifices, because they have already been posted by popular accounts and have gotten good results.