Re-use of pubkeys is similar to re-use of email addresses. We re-use email addresses for years, which makes spamming as easy as buying a list of addresses. But if nobody re-used email addresses -- instead changing their address after every message -- then lists of addresses would be pointless, because the lists would be out of date very quickly.

As best as I can tell, Nostr pubkeys are similar to addresses. But Nostr doesn't impose the restriction that email does: Nostr relays don't hand out one pubkey/address per user. Rather, Nostr clients can make as many pubkeys as they like. The trick is to coordinate change-of-address between clients (which is beyond the scope of this topic).

You mention the PoW NIP in a follow up post. PoW is appropriate when you're re-using addresses/pubkeys. I might be able use PoW for first-contact addresses. I'll have to think about that some more.