Why are women's body so sexualized?

Because the media became involved. You can thank Playboy and all the successor magazines, plus the media in general. Male interest in the female body is instinctive. But the media turned the female body into a commodity to be bought and sold. Literally, "sex sells" became the common denominator. I like many other girls cashed in on that whole scenario. I was a topless model in college. My mother was German by birth, to her going topless is no big deal. It was common at the lakes and streams near her home. I acquired the same "body confidence" if you want to call it that. I figured if a photographer wanted to pay me $40/hr to show no more than I would on the beach why not. Working part time it paid my rent for two and half years.

Because women's bodies are soft and give birth

Because straight men still write the rules of our society. 

We live in a society where women are expected to be sexualized but men aren't.

Because women spend hours making themselves look sexy.