The corollary is that most of them are, although never to be mistaken for a masterpiece, actually pretty enjoyable—if they're one's cup of tea—even as one starts not being able to remember the differences between them and they all blur together... The uninteresting ones usually don't survive this far. And so it's not necessarily unprofitable, despite the saturation. So why stop making them?
I'm not saying they have any literary merit, of course (though some do, but it's obvious that that... hasn't been the key to success in these genres). The key components of these sub-genres are being easy to write, easy to digest, and also prone to produce relatively interesting results even when the writing gets sloppy. XD