I can't understand why people sexualize nudity.

I think it depends on whether one is used to seeing people naked in a non-sexual context. If one is, then it tends not to be sexualized. If one isn't then the only association that those people can make with nudity is sexual.

All of this usually depends on one's culture, upbringing, family habits etc. Often those brought up in nudist families have no such association and are nearly always very accepting and casual about others' naked bodies.