[sidr]

I have a PhD and this is absolutely not the reason universities don't want to hire their own. In the modern day, there is no reason whatsoever that university affiliation limits the spread of ideas or collaborations. The reason is there is a funnel - only the top x% of students get the TT positions and students of prestigious institutions are just more likely to have the big name publications that forms the sole criterion for academic hiring. Even the prestigious institutions like to spread the mantra of "we don't hire our own" but this is just so that professors can avoid having the awkward conversations with their students along the lines of "you're good, you're just not better than <y> candidate".