Earlier quoted context omitted.
Often the non-technical reasons are "management doesn't want to assign the super-lead to the work, but also doesn't want to let the new hire have free reign to rewrite a legacy product. Just fix it."
I've often wondered whether this dynamic is exactly the reason tech culture is ageist. If we hired older developers, the culturally-assumed "gravitas" of age would make their relative social status to more-"senior"-but-younger devs illegible . We might actually have to sometimes give them the right-of-way on making the changes they want to make, even though they're new!
No, it really, really isn’t. Maybe 1 in 1000 cases are like you describe but the rest is just that older engineers are harder to exploit.