My personal opinion is that it costs more - hair assets can be expensive. So I figure some devs cough....Talothral...cough are just cheap Scottish bastards (like me) and so make many characters bald. <joke>To be honest we should be glad it's only pubic hair missing some Dev's have struggled and then given up even putting head hair on their Male MC's.
There's been a couple I have seen this year where the games MC went from having hair to bald simply because the Dev found it to hard to keep the MC's wig on straight