Because the government will not always be on your side, as 2020 demonstrated so well what many peoples outside the US have know from experience for decades.
This is a valid concern, but what do you say to the current system that does not prevent all kinds of fraud. Perhaps even rampant with it. An easy search will show you that large regulated institutions participate and engage in fraud at all levels of their institution. Why is it that crypto must bear a larger burden of scrutiny against the status quo?
I believe that fraud is easier to identify and financially punish on public blockchains. Exchanges and other entities already have the ability to blacklist an address's assets and prevent them from off-ramping into fiat ms being used in future transfers. Businesses exist today to prevent payments of blacklisted assets.
Compared to cash, blockchain based cryptoassets have more choice in enforcement against fraud. Also, crypto assets can be seen by all publicly on chain. It fairly easy to trace the work of cryptoasset fraudsters
If the whole point of cryptocurrencies is to work around the government and cryptocurrencies are going to strip them of various tools they use (control over inflation and monetary mass, monetary controls in the sense of taking money outside of a country, etc), governments will most likely not allow that, for pretty solid reasons.
It's an interesting experiment, I'll grant it that. But I don't think it can have its cake and eat it, too. I guess we'll have to wait and see.
The Trusty Libertarian Neighborhood Watch?