It's government's attempt to prevent things like money laundering and to generally identify the activity of people acting against their interest. The main point of crypto is to provide a decentralized, permission-less platform to exchange financial goods. This means there's not much you can do to directly regulate on-chain activities, and regulating financial activity is one of the most high-octane forms of power you can wield in the modern world, so goverments are not keen to part with it. Crypto <--> fiat currency on and off ramps are under their purview and they leverage these as regulatory choke points.
It's becoming trivial to deanonymize someone's transactions on a public chain like BTC/ETH, so If I know who you are when you hop in and out of fiat, I can trace your activity and see if you are laundering. This is why crypto mixers, which obfuscate your activity by making many complicated unpredictable intermediate exchanges, are popular. Zero knowledge proofs via currencies like XMR make on-chain activity opaque from the get go.