they do but only because clients that filter PoW are fringe. If everyone does it, spammers will create a nostr note miner asic and then it is an arms race (somewhat joking). PoW definitely provides rate limiting.

for example, if kind 0's were pow, then reply guy would have to shift strategies.

 I personally think its a multifaceted solution: some wot, some payments, some pow + some other stuff I'm not aware of or we haven't figured out yet.