In theory, Couldn't you just copy the "Crypto Name Service" smart contract that ENS and Unstoppable Domains use and make your own on-chain domain?

I'm missing something here. What is stopping someone from simply copying the smart contract used to make human-readable domains on ethereum and making their own domain tied to their address? I know Unstoppable Domains blocks users from taking big brand name domains. Couldn't someone just bypass that by using the smart contract themselves? Additionally, (I think) Unstoppable domains charges a fee on top of gas fees when users get a domain. Could this not also be bypassed by using the contract yourself?

Edit: I'm unsure if it's the smart contract for this system that inherently blocks duplicates or if it's the ENS and UD websites that track them, but could that also be a concern?

Edit 2: Does UD use a different smart contract for its domains than ENS does? For example, is tom.eth minted on UD different than tom.eth minted on ENS? I guess this has to be the case if UD blocks big brand name domains while people have already minted them on ENS.

edit 3: I'm realizing that UD does not support .eth as a domain.