It's a feedback loop.

Upper management can't tell what good work is, and thus whomever is the most shameless in talking themselves up wins, unless they royally screw up in a way that can't be missed. If you have any shred of humility or nuance about your work, you are almost certainly not management material.

I have met like 3 or 4 people that I actually had respect for in management roles, but overwhelmingly they were there solely on an overinflated sense of self importance rather than substance.