Who says they can't do that? Income taxes are quite high in some countries, but regardless of that percentage, anyone who buys or sells cryptocurrencies through centralized exchanges has made KYC and will sooner or later come under the radar of the tax administration. See the example of Turkey where cryptocurrency trading is allowed but banned as a means of payment, which means that one of the reasons why Bitcoin exists is disabled by ordinary law.
This does not mean that person A cannot send BTC to person B, but it does mean that no legal entity may sell goods or charge for services in any cryptocurrency. Every government has a number of mechanisms in place to regulate the way cryptocurrencies are used, the only question is to what extent they want to use those mechanisms.