What's the reason? What do they get out of it?
Note that I don't actually know, this seems to be the running theory as I understand it.
https://www.quora.com/Do-engineers-at-companies-like-Google-...
I put "competitive" in quotes, because you can't really have actual, honest competition if you exclude (or include fewer) African Americans, women, and people over a certain age who are "not a culture fit", as it says in the article. In that linked Quora post, it mentions a serious problem with this syndrome at Google in particular, and an internal list dedicated to it. When one is relatively young or inexperienced and one has a kickass job plus imposter syndrome, one might find it unsettling to be the boss of someone older/more experienced or perhaps more accomplished than one... (see Dan Lyons comedic output). ONe way around this is to avoid hiring anyone that distant from your own experience level.
My father was a victim of ageism before there were any laws against it. A new young CEO let everyone go over the age of 55. It was a Fortune 500 company that had been around for over a century and this tanked the entire company, they never recovered.
This company let ten percent of their staff go each year. These people over 55 were company lifers because they knew how to get stuff done. They wiped out a large percentage of their elite employees all at once. Replacing them with new hires right out of college was too much of a toxic shock to the company all at once.