@twinklecake
That's a cultural trait. The Western world, US in particular, are individualistic countries. Those praise uniqueness. Japan is a communal country, as are most Eastern countries. Those praise execution. Here in Western countries, it would be a death blow to publish something that is such a close imitation of something else. In Eastern countries, it's not an issue and it is how well they execute on the story that matters. A great example is the author of Fairy Tail, who continually remakes the same manga with different elements and names. It's about the pursuit of perfection rather than providing a novel idea.
That's why you will have dozens of mangas that revolve around the same premise, have similar characters, similar thematic elements and more being published concurrently with no one caring. I like it. It allows me to sift through multiple stories that don't quite hit the spot for me until I find the one I like. In Western media I have to just come to terms with the elements of a story I don't like to enjoy the parts that I do.