I agree there are multiple reasons contributing to why there are so few such high numbers/low SES applicants in their applicant pools. One is just not that many such people get such numbers. But another is a lot of such applicants are not necessarily aware of their potential opportunities, including their potential aid if admitted.
So, these colleges are pretty much all engaged in various outreach and targeted marketing efforts to try to get as many such high numbers/low SES applicants to apply as possible.
But I think it remains an important practical observation they are doing that precisely because they think they can only compromise their normal academic requirements so far for low SES applicants, and they are finding they are not getting enough low SES applicants who fall even within relaxed requirements.
I will finally note this was actually one of the main topic of the now famous/infamous Dartmouth/Yale discussion in which their respective Admissions Deans talked about how much they valued getting test scores despite being test optional. One of their points is they wished they got more test scores from low SES applicants even if those test scores were below their normal ranges, because they were willing to give contextual consideration to such scores, meaning if they were sufficient outliers for applicants with such backgrounds.
Again, all that is independently interesting, but it further supports the observation that these sorts of colleges still view low SES applicants with sufficiently high test scores–even on a relaxed standard–as scarce, such they wish they got more of them.