The logic is simple.
Why keep somebody who is 56yrs old, beginning to show health issues and making twice the salary of their 26yr old replacement?
And tops out at 40hrs, when the kid is willing to do 50-60.
In some cases the extra skills/experience makes the difference. In many others it does not.
Reality. It sucks.
The reality is rarely that simple. In tech, the pay for a 26 year old isn’t that much lower than a 50 year old... hell... sometimes it’s higher. In my case, four 110k millenials couldn’t do the job of my one 140k salary. Numbers didn’t benefit the company.