bitcoin IS decentralized and free from middleman or third parties. the fact that some people choose to trade bitcoin and do that on centralized exchanges does NOT change this fact.

as for why they choose them is mainly because no matter how much you try some people will never want to change so they will keep going back to their centralized ways that they are used to. any other market is centralized like stocks market so they want to have the same feeling so they choose centralized exchanges. these types of people are only in here for profit making.