They don't there are smaller NFT platforms that offer NFTs of art by smaller artists and there is a ton of fraud occuring on those as well. People misrepresenting themselves as the artist etc.

I'm sure I'm just being naive, but why wouldn't it help? Aren't NFTs used to securely show unique ownership of some item?

The knockoff seller also has unique ownership of the knockoff. It doesn't fix anything.

You don’t really need a block chain, you can just register serial numbers on a legitimate website. This was solved early on with cd keys.

NFTs don't show ownership of anything, at least not in any legal sense of the word.

Not an NFT, but blockchain approaches have been suggested for proof-of-invention problems.

What is the point of a blockchain where this could be achieved with simply a public/private keypair.