There are many industries around the world where agism is rampant, and there is only ever one excuse: money and control. Young people are cheaper and more gullible.
It's not a secret, far from it, it's one of the few things for which the cliche "everybody knows" actually applies.
Sure, many industries try to cover up this fact a bit, but only tech makes up such elaborate stories to pretend it isn't all about cold hard cash.
Obviously, there is a reason for this beyond creating a flattering self-image: Most other industries that exploit the young and naive have plenty of supply.
Good engineers however are rare at any age, so market forces aren't sufficient to maintain this exploitative relationship, it needs to be artificially enforced.
Resulting for instance in the insane situation that many older engineers are unemployed whilst the industry is importing cheap young and easily abused engineers from abroad.
(Full disclosure: I too used to believe the main cause for that was that these older engineers had outdated knowledge and no longer making the effort to learn anything new, until I became a hiring manager. And although I have nothing but anecdotal evidence (so has the other side btw), I will be so bold as to state that that is a big fat lie.)