Because more customization costs more money. Because Blizzard wanted visual archetypes for each class and still gave us more control over character customization than any Diablo game previously, and more than most ARPG's in the genre allow for.
Like, it's fine if Blizzard wants to make a game where not ever character makes you (the royal you) rock hard and aroused, IMO. Would I like more customization options? Absolutely, but framing it in the way many do of, "I can't have sexy ladies" is just a kinda cringe and gross way of going about it IMO.