It's more of laziness and convenience for the devs.
The convenient part about the set and rigid body proportions is that it's easier for them to model gear on them. I heard that's why the lower bodies of female Miqo'te got reduced.
The males 'lacking' nipples is just a Japanese preference, I heard they think it looks unattractive. They do have nipples however, but it's not a darker tone from the skin color.

Although I have some problems with the body models. They're not bad but they look very rigid (especially the males which lack curves) and both genders are lacking in certain areas.
The height slider also just scales characters instead of actually making their limbs or body longer. Max height Lalafells would have big beach ball heads and it's super creepy lol.