I genuinely think both galea shape and neck alignment play a significant role. When I say neck alignment I mean more than just forward head posture and a lot is not externally visible. It's very hard to describe but even someone with non forward head posture may still not be internally aligned, ie the neck 'juts out' very low down nearer the top of rib cage.
I believe the occupilatis and Sub occipitalis muscles play a big role. My thinking is chronic tension in the neck muscles particularly the SCM and upper traps can transmit through the interconnected scalp muscles (occipitalis, temporalis, frontalis). Over time, this sustained tension on these 3 areas tightens the scalp, reducing circulation and promoting low-grade inflammation and fibrosis and increasing local DHT sensitivity and possibly DHT production. This matches Norwood pattern almost exactly. The skull shape is either related (bone changes / expands as people age due to the above) or simultaneously adding to the problem.
I think there's a lot more to consider but imo most is also mechanical, jaw tension for example.
Even if I'm wrong on some of the above it's impossible to deny in real life looking at people's head from side profile and the way it 'sits' in alignment to the upper body, correlates to good or bad hair.
For women I don't see the same alignment issues I'm trying (badly!) to describe, even those with poor posture or fhp it doesn't look the same.
I'll try and explain better with pictures at some point.