>>12320750Biologically speaking God is lazy. You've got to have something connecting your shoulders to your belly, it's 100% required for at least one parent to have working mammary glands, nipples don't cost any more to build than normal skin so it's simplest and easiest to just lay down the same tissue in everyone. Otherwise you have to evolve new growth factors and receptors that would deactivate nipple growth and get them onto the Y chromosome but there's no benefit or evolutionary pressure to do that so it hasn't happened.
