This is simply because the existence of these third parties is mainly related to the fact that exchanges are made with fiat money, whose identity cannot be hidden in any way. Peer-to-peer platforms can become completely decentralized if exchanges between users are carried out using crypto only, but users of these platforms are often in countries that do not support cryptocurrencies, so they are looking for local sellers so that they can use their local currencies to complete buying and selling operations. It means that the platform plays the role of the third party.
The problem with this third party is not that it gets a fee for the service, but rather it is the powers that it has, so it is able to possess the users’ private data, and this is what must be done to avoid it. I do not see any harm in the platforms providing third-party services without having the ability to track user data.