Let’s say every lemmy instance has a “memes” community, and you want to find them. Unlike DNS, where there’s one authoritative record for memes, to find all the different communities for memes, the local instance would have to send a search request to each other federated instance, wait for the results (what if the remote instance is offline?), figure out how to sort them, paginate them, and display them to the user.
Would it be possible? Yes, but it’s probably more of a headache than the devs want to implement. If you check the issue tracker, you might find some discussion about it.