females are soft, for child bearing.
Are you imagining that undeveloped muscles make the human body more stretchy? While I can’t speak to pregnancy and giving birth because I’ve never done either (and am probably approaching menopause anyway), as someone who works out and keeps up some muscle mass because I like the amazon look, that seems highly dubious. If anything, a properly toned body would likely help, as it would make it easier to carry an infant around both before and after birth, be more resilient against birth-caused physical damage (not so much with the chance of vaginal tearing, but with everything else), and make it easier to recover from the stresses of pregnancy, birth, and breastfeeding by having protein stores. That being said, one does still need fat stores as well, so a stereotypical “bodybuilder” lean muscle physique wouldn’t necessarily be a good idea if you want to get pregnant. (Though that does lower your fertility, which is also helpful if you don’t want kids.)
I mean, if someone wants to go for the “soft look” (whatever that means to them) or are practically restricted to it due to their biology, no shame in that. There might even be some advantage(s) to being non-athletic which aren’t obvious to me (and given that nearly everything in life is a tradeoff, I would hardly be surprised) but even if there isn’t, modern medical care mostly offsets that. I will also point out that until fairly recent times, poor women were at least somewhat in shape due to needing to engage in a lot of physical labor (if still often disadvantaged due to lack of adequate nutrition and medical care), so the “soft” middle class woman is kind of historically anomalous.