I guess we cue to the other discussion: getting tenure is a relatively rare event. It's not like college basketball players going pro, but it's close. And of course plenty of people who went to grad school don't go on to even try to be one. There's somewhat more than 2.2M grad students in the US at any given time. One stat I saw is that about 500k Master's degrees and about 50k PhDs are conferred in an average year. There's 1.2M "college faculty" (whatever that means) in the US, and about 45% or part-timers. I've seen numbers that say there are ~280k tenured professors in the US.
I think it's easy to make the case that the vast majority of "people who went to grad school" aren't anywhere near being a tenured professor or even on their way to trying to be one.
/jordan