The reason might be due to other people seeing it as wrong and don't like it at all, there were those 3 dudes in that one cafe making fun of them for doing it.
I contradicted myself, I am stupid and half asleep and forgot they were the same people in this chapter. Though, still, some people might see it as wrong, and probably a lot of people like their own original appearance, so that can explain why it ain't a utopia.
Also. this might come at a cost for sure, so not everyone has that money...
To be fair, there's only so much strength you can pack into a child's body without making it look like some dwarf macho's. And that may not be what they wanted, when they chose to remodel their body.