Exchanges are certainly not decentralized, as you say. As many people say, don't leave your funds on an exchange.

Cryptos are though. Having control of your private keys is one of the first things everyone needs to do if they hold significant funds. The taxman is going to get us anyway regardless, by the way, and that won't have anything to do with decentralization or not, because you are legally bound to report property values. Whether or not it is enforceable in certain circumstances is up for debate though...