Certificates still have the problem of who your trusted root is, and don't (by themselves) solve provenance issues - if two people have certificates for the same bicycle then there's no way to choose between them, whereas an NFT inherently has an auditable chain of custody going all the way back to the original issuer (presumably the bike manufacturer in this case) and digitally signed at each stage.

I'd love it if these industries/communities adopted log-structured distributed datastores with digital signing to track provenance of things without wasteful "mining". But the fact is that they've shown no willingness to do so, and it's a lot harder to bootstrap a system like this - if your system relies on a trusted root then you need the trust before you can start to develop the system at all, but users have no reason to trust the system until it's all fully running and proven.

It seems like it would optimally require the manufacturer to participate in creating a token representing that serial number and assigning it to the person who bought the bike who would have to transfer it to the next owner and so forth.

And so when I sell my bicycle to someone (and write them a receipt) but forget to do the weird token transfer thing, who owns the bike?

In the eyes of the law, the guy holding the bike and the receipt owns it. What the NFT says is irrelevant.

So how are NFTs useful again?

If they had been out long enough to be pervasive you could require all online marketplaces ebay,offer up, facebooks etc which are commonly used to traffic stolen goods to upload an NFT as part of the transfer whose description matches the item being sold which is transferred with the good.

It would be like requiring every good sold to have a trivially electronically verifiably accurate original receipt that only someone who acquired it illegitimately would lack.