I'm not sure. There may be a specific allele for them, but on the other hand, maybe it just has to do with how generally "hairy" you are, i.e. men with lots of chest and/or back hair as well as hair on their arms and legs tend to have thicker eyebrows?
Of course, that hypothesis can't be airtight. My husband isn't hairy at all except on his legs and arms and he has beautiful thick eyebrows. They aren't long, though, i.e. the individual hairs, and not curly so he's never trimmed them. He has thick, long, luxurious eyelashes too. Ah, and here's me with the eyebrow pencil and mascara! :)
Thick eyebrows and eyelashes, and also large eyes, are very prized in West Asia. If they don't have them they pencil them in. I don't know if they grow more and are trimmed as they get older.
It's definitely not just a West Asian thing, though.
I can definitely understand if these men cut them, but clearly they don't.
Donald Sutherland:
John LeCarre:
King Charles has a bit of that going on as well, I think.
Maybe with age they grow more, like noses can sometimes mysteriously seem to do, or ears on men in particular.
Now, if they looked like this, it would be a pity to cut them or do anything to them, imo.
To each their own, but I don't like the tweezed look which was in for a while. Perhaps I'm just not used to it.