For example, imagine if most baseball teams started excluding non-white athletes again. If I owned a team, I'd immediately start hiring those athletes; I'd have by far the best team in baseball in short order (EDIT: To be clear, I'd have the best team because I'd have the market cornered on a large portion of the talent; it wouldn't be because of some imagined (and false) racial differences in performance.)
But in business (and in sports) it never works out that way. Certainly some executives have discriminatory attitudes, but that doesn't account for all. Certainly some will give into and/or are more exposed to social pressures, but not all. With the very high demand for talent, why aren't there businesses snapping up the older developers?
(There also is another problem: Excluded groups tend to avoid that marketplace. They are discouraged by their parents, teachers and peers, and they may not want to have to deal with discrimination every day for their whole careers. But that shouldn't apply as much to middle-aged developers; that labor supply is already in the pipeline.)