Here we are with that elitist mobility
. . .though I had much the same thought!
I am by no means proficient or super strong in the front squat, though it has become my go-to squat. It just feels better for me than back squats; there's something about the placement of the load and... the shoulder position.
I always hear people talking about front squats taxing the upper back and I simply don't understand. If you want your elbows to drive forward and up, you spread your shoulder blades apart, wrapping them around the ribcage (which would mean lengthening the rhomboids and mid traps, not contracting them). That alone got me from a semi-uncomfortable front rack position that required multiple sets of front rack mobility drills to one that I can do with hardly any "mobility warmup." So....I guess I don't understand where the "upper back work" in front squats comes from. I feel almost zero work in my upper back, because I am not trying to squeeze anything up there.
Unless by upper back you mean "upper traps" or something? I don't know what I'm missing. Maybe I just haven't gotten to a point where it's heavy enough to cause something up there to kick on?