PoW is essential to Bitcoin but PoW doesn't stop spam, it didn't take off when it was Hashcash and it also doesn't stop spam in Bitcoin, see inscriptions for example.
> then why aren't they spamming nostr right now while the cost to do so is absolutely free?
They are. When I turn off the Web of Trust filter in Nostur I see spam all the time, if you are not seeing it you are relying on your relays doing the filtering for you. Add relay.nostr.band and you will have spam in many replies (with WoT filter disabled).