The environment. See many photos of tribal villages where women don't cover breasts.

Or on the other side the not that distant outrage about women wearing trousers which exposes leg shape.

Tribe women not covering breasts does not imply tribe men did not get excited to see them. Modern women, specially young, dress very provocatively sometimes, and men definitely feel excited when they see them, yet they do it anyway.

One of the oldest pieces of extant textiles is a string skirt, which was apparently designed to reveal as much as it hides.

I find it entirely plausible that dressing in string skirts and flowers while dancing to songs with highly ambiguous lyrics would have been the neolithic equivalent of "woo girls" tottering around on heels, intermittently pulling down skirt hems and emitting the Sorority Mating Call ("I'm soooo drunk").

Koyo, to wa terashting mali? Fo to du seping kori mi. (Baby, are you an attitude control Epstein? Because you make my heart spin.)

Bonus clip: https://www.youtube.com/watch?v=m_-Qtz70_z4

    Push wit xante
    Push wit lek
    Leta kom but ere dek
    Push wit xante
    Leta kori fo du mi xush

Could it be that distant tribal villages didn't go through the same type of evolution and their brains didn't develop specific receptors in their visual field/cortex to detect nipples and bind them to the reward system in the brain? We know that even a single neuron in visual cortex can detect complex objects shapes like animals, who tells us that we didn't develop a specialized nipple detector neuron linked directly to the reward center while Papuan tribes didn't?