The simplest explanation is that

a) humans are visually orientated – a woman’s musk might be obvious to a dog but not apparent to a male human unless they are very close
b) humans typically walked around in grasses etc that hide the lower regions

Having a sexual signal on the rump is not going to work in that situation. You want it high up.

Compare a female gorilla with a human woman.

When they are not lactating, they basically do not have much of a visible breast.

Gorilla females do not need to advertise as their lives are relatively static. They will mate with the big male. They do not compete with the other females to do so.

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I have always wondered how women with large breasts protected themselves from predators.

Large breasts limit mobility. You can’t run, jump, sometimes, in the civilised society, it is hard to even lift our hands high up.

Imaging having to run from predators to save yourself.

So, I guess, by natural selection, it is a trait that should have disappeared, as these women are quite vulnerable to hunting predators.

However, over the course of evolution, when humans started cohabiting and moving towards the top of the food chain, this problem could have been easily solved by defending the weak.

And obviously, large breasts are a sign of fertility.

Fat deposition. Fat in those days are a sign of luxury, well fed and resourceful.

That woman must have been protected all her life, which indicates, safety and a strong survival instinct of the group.

And, beyond feeding, large breasts also work as a cushion to comfort infants.

Crying babies are a pain.

So, these women were assigned to take care of infants of the tribe. Growth again.

This must have made them desirable for mating.

And overtime, because of natural selection, they must have gotten bigger.