IMHO, it's because organizations don't really value diversity in the ordinary sense of the term. They value proportional representation of all demographic groups that have legal standing to sue them for civil rights violations, and who have organized interest groups who stand ready to do so. DEI exists to align the organization towards this objective.

It's not just about organizations though. Whenever I hear ordinary tech workers, non-managers, just talking informally about underrepresented groups, they rarely mention older people. It's a forgotten group (kind of like GenX in general LOL).

I should say, I don't want to pit underrepresented groups against each other. As far as I've seen, various forms of discrimination go hand-in-hand. I once heard a prominent businesswomen say that the most sexist workplaces she's seen were also the most ageist.