The term "free software" predates "open source" by many years. Stallman started work on free software (the movement and the code) many, many years before Eric S Raymond coined the term "open source".
The terms "free software" and "open source" were created very deliberately to express different values. The conflict between those values has been the source of a lot of contention in the past. Nowadays there's less of that, probably because as the industry grows, the proportion of developers who are old enough to remember this stuff is going down.