Because the training in "deep diving into something" is evidence that the person can learn some other thing over the next few years.
It is a tenuous connection at best. It works somewhat in STEM because of the mentioned deep trees: some people are just naturally curious and will get to the bottom of anything, be it DS&A, circuits, complex analysis, or accountancy.
Because in most western(?) states, the entirety of education and (junior) hiring is just a load of bs.
I totally understand too, it’s super easy saying “oh for this position we require X, Y and a PhD in Z”. It’s guaranteed to filter out lots of candidates. Whether that filter is the one you need is another question altogether.
They seem to think it is.