Why would Bitcoin purists care about off ramping onto fiat?

This seems awkwardly unnecessary for a technology that has only prioritized deflationary economics and economic sovereignty through private key encryption.

Serious answer: Because they have to eat. Being a purist doesn't mean they can afford to ignore the world they live in; even if they believe that USD is fundamentally worthless and keep all their wealth in Bitcoin, they still need to occasionally pay bills to people who don't take Bitcoin.

There are virtually no actual purists. And criminals are generally more opportunistic than ideological.