Wouldn't the same also apply to preferring the shaved head look over a more natural male pattern baldness look when those are your options? Why shave it? Who cares? Does it matter?

More directly, at the risk of a handwavey evo-psych just-so story: Hundreds of millions of years of evolution, perhaps? A ton of characteristics driving attraction are signals of health / youthfulness. Weight, musculature, nice skin, good teeth, etc. And yeah, good hair! Male pattern baldness is definitely associated with aging even though many people will probably spend more of their lives follicularly challenged than they did with good hair.