This is because NFTs don't actually understand the concept of a "sale". They can be transferred to another wallet, but this wallet may belong to the same person as the previous wallet in which case paying royalties wouldn't make much sense.

Paying royalties is entirely optional. What's built into the contract is a calculation of a value for royalties to be paid, but it's up to the marketplace to use it. Other platforms already ignored it.

Smoke and mirrors. The exchange platform holds keys that allow them to modify the behavior of the contract.