Variety is fun. For some of us, playing around with sliders isn't just something to get out of the way before you get to the game. It is itself fun, intrinsically: a place to experiment, be artistic and creative, role play, and take satisfaction in the fact that no one has ever played the game with this particular character before, in the same way we find it satisfying to play on a random world no one else has seen before. My first hour (
literally, I clocked it) of playing Fallout 4 was spent in the character creator, despite the rest of the game not holding my interest for long.
Swapping between heads that have gotten an artist's stamp of approval guts most of that fun. Don't forget, people make bases that aren't professional looking or performant, and share images and videos of them, all the time. And you let players do this, because it's fun and this is a sandbox. You do your best to make the pieces look good and perform well, but with few exceptions (like a minimum distance between trees) you don't limit what players can make.
...With that said, the greatest need for variety is not with the player characters, but with the zombies, and eventually bandits. If a new player starts a game and plays just through the first Blood Moon, they'll still typically see hundreds, possibly thousands of zombies. And they're only getting a subset of the zombie types at any given gamestage. So if you do any further work on character variation, I hope you do it on zombies. Tinted clothing and skin, subtle scaling, body part swaps... I get that UMA didn't pan out, but anything would be better than clones - even new, high quality HD clones.