Because it’s illegal in many places. A female exposing her nipples in public is considered an act of public indecency in states like Illinois, which is a misdemeanor[0].
Edit: not that I agree with this law. It’s a very clear double-standard with roots in conservative Christian thinking.
>A female exposing her nipples in public is considered an act of public indecency in states like Illinois, which is a misdemeanor
This is a very bad comparison. That's like saying "drinking on the street in public is illegal in many states, so showing any visual references to people drinking on Facebook is potentially illegal and should be removed".
This existed thousands of years before the first Christian, and in almost every culture on the planet.
You might want to re-examine your biases to figure out why you assume that things you disagree with are automatically "conservative Christian thinking."
Edit: not that I agree with this law. It’s a very clear double-standard with roots in conservative Christian thinking.
0: https://www.menshealth.com/sex-women/a19545146/nipple-double...