This is quite a complex explanation of a process that I see as pretty basic and linear and as old as homo sapiens: new people are exciting, old tenure-wise people are usually not exciting because they are not new people, and their importance - especially in a big 1000+ company - is often overstated. The ship carries on anyway.
You can see this phenomenon everywhere and it is as clear as Caribbean water in marriages or relationship: one can be married to the most good-looking, engaging partner around, but doesn't the so-and-so looking person you have met a bar after a few drinks feel like a rush of life? After choosing the rush, some of them have regrets, but it appears that, in the vast majority of cases, life goes on. And it surely goes on for a company, so companies are not very motivated to spend more money for someone who's old news.
My experience in consumer and enterprise tech tells me that most the rationalizations one reads on HN and similar forums, that is the cost-benefit analysis of hiring or retaining, the calculations of how many hours of engineers' time are spent on hiring (or meetings!) instead of working on product, are fun academic discussions as much as doing a cost-benefit analysis of marrying your partner or going on as is, or getting a dog. They all talk about them, but how many do those analyses? How many go by feelings instead, or aspirations?
For example, in my current company (but isn't it the same in all companies?), when somebody leaves, there are the customary "I wish the very best in your future career" and when someone joins there are the customary "It is so exciting to have you here, let's book a 1:1". Opportunistic messages, but they also say something real about how we feel about the old and the new.
I remember a Uber guy who left the company after a few years of tenure. On Twitter, they wrote the usual: "It's been great to be here, I met so many exceptional people (...and all that customary boredom)". After 6 months, another Uber-er wrote on Twitter: "We worked so well on that product, I did not know you left!".
It was an enlightening exchange.