There are people complaining that Lemmy shouldn’t copy Reddit (I say, why not?)

Assuming you mean copying posts from reddit: Because, without the person that originally posted the question or topic, it feels like there's little point in discussing the topic. I was subbed to a cycling-related community that copied every post from an equivalent reddit sub, and it had zero comments. I'd start to write a comment from time to time and it was like, "What's the point? OP isn't going to see this response."