You don't need to use centralized exchanges. In fact you aren't even using crypto when you use centralized exchanges.
Not using an exchange does mean it's harder to enforce, especially at scale, but "not getting caught" is not the same as "not committing a crime". And I have no sympathy for all the people e.g. trading crypto and then getting all surprised when they owe capital gains tax. They thought it was "just math" and that "math is law".
Turns out math isn't law.