[Old topic, but interesting.]
There are aesthetic reasons: No hair all around seems to highlight the shape of your head, which is or at least can be beautiful. Especially if the hair at the back and on the sides of your head is relatively long and clearly visible, it changes the shape of the head at least in the viewer's perception, making it broader.
But most of the reasons are connected with the fact that the horseshoe clearly marks baldness as MPB and thus as not a stylistic choice. It is therefore associated with (old) age. And if you start noticing that you are losing your hair, this is the worst-case scenario for you (exemplified by your dad, your granddad, your uncle, anybody whom you might think very old and very uncool at the time).
The odd thing with me is that I tested a full shave, but I was not really happy with it and did not feel to comfortable with it aesthetically. Now there is a horseshoe, though cut to 3-4mm, with a relatively big shiny top. If you had told me that this it what I would look like thirty years ago (with relatively thick glasses as a further element dreaded in my early teens), I would have collapsed. But oddly enough, I'm fine now. Perhaps it was the invention of the digital camera, where you can take a thousand pictures of your head and then find four that you like and you decide to take these as the starting point for the rest of your life.