Finding native speakers of languages like Chinese, Russian, Farsi, etc. who are also eligible/want to have a clearance is a challenge (it’s expensive and self-limiting, since US citizenship is a requirement).
Training people already cleared in those languages takes a ton of time, expensive, and yields linguists with mixed-usefulness (think understanding formal Spanish taught in highschool versus Spanish actually spoken amongst peers/friends). There’s slang, intonations, etc. that non-native speakers have to spend time learning/may misunderstand.
In other words, OSINT has a much larger talent pool that yield arguably/presumably better translations.
I think the important part of this is how the vast bulk of the OSINT we're discussing is of Americans not suspected of a crime.