Depends what you mean by altrusitic. These schools are doing these mailings not as a public service but because it serves their interests, I agree totally.
What are their interests these days? I’d put at the top, finding the best collection of students as they define, which itself is a complex rubric including academics, talents (athletic, artistic, musical, literary), diversity in SES, race and geography, prospects for future leadership in their fields (political, academic, business), current and prospective monetary support of the school, alumni engagement (for those with a legacy preference). Having more applicants to choose from allows you to better “shape” the class.
Enhancing the brand is a separate interest. Here we are talking about combatting the negative perceptions about an institution that perpetuates privilege for the privileged while sitting on in some cases billions of dollars in tax advantaged endowments. The current push to admit more Pell/lower income, first gen and racially diverse groups is in service to both collecting the optimal student body and combatting negative PR perceptions. Reducing the admit rate is pretty far down the list for most elites IMO.