There’s no reason to sexualize the breast. They are baby-feeding organs.

Disagree. As with other body areas and organs, they can be multipurpose.

AFAIK the other great apes and other mammals don't sexualise mammary glands, but in humans they do have a lot of extra sensory cells, and that can easily tie in to sex, mating, attraction. In humans they've also evolved a (somewhat unnecessary) layer of extra fat cells expressed in the female in order to better-resemble the buttocks, according to some theories.

Given the unusual helplessness of the human infant and its long development time, sexual attraction between partners is emphasized in all kinds of ways, and unlike most animals, sex is something that's on the table all year-round.

Fun fact (updated): male breasts can evidently become functional in some circumstances.
https://www.scientificamerican.com/article/strange-but-true-males-can-lactate/