Because it’s well known. 
Ucb created the FAQ because the question was recurring so I suppose colleges could be clearer but guidance counselors who know what they’re doing should know that, it’s very basic. I understand that many public school gc 's aren’t that good, though. That’s why, I suppose, you need specific proof to show the gc?
I don’t know what colleges you’re interested in and don’t have time to list all one hundred of them, but if you want random colleges to show your gc that level reached is what matters in math and foreign language:
Email Hamilton, Harvard, Yale, Princeton, Tufts, Amherst, Middlebury, Haverford, Mount Holyoke, Smith, Macalester, Dickinson, St Olaf, Grinnell, Lewis and Clark, Hendrix, Davison, Emory. Being 'competitive’is what you need to know. And if your child is on the Junior Olympics team or will publish research in Neuroscience by all means add that.
The “level reached” applies to all of them. (IE., if you had level 3 in 10th grade, you have completed three years of the admission requirement, not two; of you had AP foreign language in 11th grade, you have completed level 4+ of the admission requirement, not three.) if by happenstance you email one of the 100 colleges and they tell you differently please post their name and complete reply here as this would constitute a big change in a longstanding policy.
As you already acknowledged, individual circumstances are taken into account. “I don’t like Spanish” is not an individual circumstance (nor are ‘I don’t like math’, ‘I don’t like history’, or ‘I don’t like science’).