Because there is no reason to. I have not used a razor in 52+ years and I’ll be damned if I want to start now just because someone thinks it looks odd. Someone once suggested I turn my head upside down because I am far from bald underneath.
Not bald nor balding, but I also don’t see why anyone would feel the need to go “full bald”.
Ultimately, the goal with your haircut, no matter how your hair grows, is to look as good as you possibly can.
Combovers don’t look good, regardless of whether you are balding or not, as our current President proves.
Longish hair, when bald or balding, does not tend to look good (though it can be a bit individual specific).
Full bald is almost always fine, unless you also have no eyebrows.
Trimmed bald always looks good (see Patrick Stewart).
So…if it looks good, then why not?
Actually, “bald by choice” is a thing. There’s at least one forum all about shaving your head (I participated for a year or so when I was bald by choice). Shaving my head was a spontaneous, weird thing to do to show support for my gf’s cancer (which never even needed chemo, so i shaved for nuttin). I kept shaving for a while to enjoy the sensations, like throwing cold water on my head on a hot day, intense. And in bed, it was like my gf had a new guy. Women (and an alarmingly number of men) liked to rub my head. That was a positive or a negative according to who was doing the rubbing.
But shaving my head twice a day, every day, got to be a bit much. Hot towel, exfoliative, rinse, shave, rinse, witch hazel. Can’t believe I did that for 3 years.
Bah just leave what grows grow and what doesnt well heh its bald right?
The rocky horror Riff haircut is quite fashionable 
You know what sucks about baldness?
Not the baldness.
Its the lack of something to stop the sweat from making a beeline right into your eyeballs
This was me until about a month ago, except for a few wisps of random hairs trying to hold-down the fort front and center. Thanks to my ancestors and their genetics.
I decided to try shaving my head (I have a thread around here, somewhere). My main problem was the newly shaven area had never seen the sun, so was as white as a lily. It has taken some time to darken some and blend in with the rest of the scalp. I now keep it either shaved or very short (see Matt Lauer).
It has taken some getting used-to on my part, for sure, since vanity is in short supply for me, and I have been married for a long time and don’t need to impress anyone. But I think it looks more like I own it now - it looks like I shave my head as opposed to looking like I am going bald, IMHO.
That’s #2.
I really miss the millisecond warning that you are going to hit your head. It least once a year I end up gashing my head on a tree branch or piece of equipment at the gym…
But not a need.
If you have a combover, then you need to do something about it. If you look like Patrick Stewart well, sure, you could go full bald, but there’s no need to.
Yeah, this is my reason also. As I job hunt now and then, I think it would give me a better shot at a job as all bald doesn’t mean old anymore.
I’ve read discussions where people discuss their need to shave their heads. They say that they feel powerless to stop the hair loss process. They are unhappy with their partial baldness. Shaving makes them feel that they once again have control over the situation.
It’s a common thing in the bald by choice community.