I think this is just a logical conclusion of the US's wider mentality towards higher education. It seems to me that the US considers the main purpose of a high quality college education is to produce good economic and cultural elites. Thus, the focus for these colleges is on identifying and fostering 'good citizens' - those that have the highest potential to be shaped into a well-rounded and ethical stewards of this economic and cultural capital. It follows that these citizens are to be drawn from and reflect the diversity of the US.

I suppose the thing that's odd is that - uh - education and academics feels very secondary in all of this?

For example, for college admissions in my country (the UK), you take national subject-orientated coursework and exams at age 16 and 18 on topics like History or Chemistry, write an essay about the subjects you want to study at university and then maybe have an extra exam or interview that's run by the university. And we're not even a particularly academics-driven culture. I'm sure other commenters can more succinctly describe the US college admissions process, but it really does seem to elevate the importance of (1) extra-curriculars + cultural + social action (2) high school class grade percentiles (3) standardised 'IQ-adjacent' testing and (4) motivations and outlook. What's missing from this list is identifying and developing the next top wave of scholars that might have a narrow but deep interest in an academic field.