In the field I specialize in (I'm currently a moderator of crypto-SE), I see many obviously pointless papers published, often in publications where that costs money. I imagine that's the same in many fields. Why are such papers published, or worse, studied?
Example: there are tens of thousand papers on image encryption, including thousands at MDPI, which has Article Processing Charges. Yet, the subject is almost¹ enough to tell the work is pointless: once digitized, and especially after it's compressed (as almost all images are today), image is data that can be encrypted just like anything else. The International Association on Cryptographic Research correspondingly publishes no paper on image encryption in its peer-reviewed publications; many of the papers about image encryption it references are refutations, and they do not go further than its non-reviewed preprint server.
Worse, I regularly see evidence that students at least think they have no choice but study a pointless paper riddled with errors. In this recent example, the paper attempts to apply an (unstated) Elliptic Curve signature scheme to inter-vehicles communications, but examination shows that the authors failed to find parameters for a usable Elliptic Curve. The paper cites 36 references on the same vein, many comparably poor. The OP ended up writing (in chat) "unfortunately I have no other choice but to study this paper", and I could not determine for sure if it was designated to them as a reference, or was just a poor choice.
Addition: what about the hypothesis that some subfields like digitized image encryption are abandoned as obviously not worth attention by actual reviewers, and flourish precisely for that reason? This particular subfield is remarkable in that it has developed standard patterns for the many articles, and some bogus measures of the efficiency of the encryption. To see what I mean, sample the papers this query returns (at least these all are officially free to read, if not to publish).
¹ I admit we must check that the image is encrypted and decrypted by a computer, and that the sole objective is encryption (rather than e.g. hiding data into an image, that is steganography, or Thumbnail-Preserving Encryption). Examination shows that most articles returned by that query are only about encrypting a digitized image, and many of the others are seriously defective.