Why? When I post a link and at the same same formulate some opinion on it in the body of the thread, and you do the same with the same link but in another community and with your own opinion, then that's still also a crosspost (because to Lemmy we both linked to the same site, it doesn't care about the body of the thread). Why should those two share the upvotes/downvotes?
Let's take an example. You have news about Biden. You post it in some community with probably your pro-Biden opinion along with it. Then you have someone else posting the same link on c/conservative with obviously an anti-Biden opinion attached to it. Why should the two mutually promote themselves?