The thing that makes something like bitcoin decentralized is that it's built on consensus between participants: when you make a transaction, you publish "I'm sending $x coins to from $y to $z", signed with your private key, and once enough participants agree to add your transaction to the chain, it's "real". This is why bitcoin transactions can often take radically longer than other forms of payment to resolve. NFTs work the same way: You say "I'm paying $x from $y to $z to buy this picture of a monkey".
If you were separating the payments from the items, you need some entity that agrees "yup, nytesky paid for this item and so they own it". And once you need an entity for it... you may as well skip the blockchains and just use a database.
For what it's worth: this is functionally how most NFTs end up working. The entity selling them just takes payment via ethereum or other coins, but at best what they're storing on that chain is a pointer saying "oh yea, nytesky bought this thing that we store elsewhere".
If it's starting to sound like a big scam... that's because it is. "NFTs" is basically just a fun buzzword for "we figured out how to hype up and grift assets without actually having to like, have assets to sell. We can just generate them out of thin air and then convince gullible people to drive the price up".