NFTs are not semi-centralized because Web3 or blockchain. They are like this because our current notions of 'ownership' are
intrinsically centralized. Social reality beats tech, so the tech will end up either emulating it regardless of underlying 'decentralized' technology, or the tech ends up withering away due to non-use.
Do a random sketch on your computer. Now put it on some Web0 site. What does 'ownership' mean? You still have the sketch after all, no matter what people do with the sketch. Inasmuch as you 'own' it, you may want attribution (people giving you credit), copyright (both limits on copying and copyright fees), warranty protection (if someone looks at the sketch and gets an epileptic attack, it's not your fault), ability to transfer ownership, etc. In short, the current notion of 'ownership' mostly related to relations with other humans*.
All these are provided by the law, and can only be provided by the law. 'Code is law' simply changes the entities making decisions, it's still law. If a single chain is a law, than it must have only one record for 'owned' stuff. If you had multiple chains, this strongly affects the value of ownership. Imagine multiple records of ownership - does one pay copyright fees to all the different records? Split them? Recognize only one? Any solution ends up devaluing or centralizing.
This is a poor match for NFT tech, since nothing really stops you from forking (a cryptocurrency fork has serious roadblocks. A NFT fork has a much easier time fighting the regular NFT using the NFT's own assets), but to have value one needs to approach a single source. So NFT will either fork to nothingness or end up de facto centralized.
* In a physical asset, there are a range of decisions regarding the physical asset, but again the question of who makes these decisions is a question of relations between humans, and inherently pushes towards centralization.