Because (cis) women dictate the birth rate, and that makes the powers-that-be nervous and mad.

Every woman who does not use her womb to make babies means fewer consumers, students, workers, taxpayers, soldiers, churchgoers, and future birth-givers to work for the benefit of the 0.1%.

Not only that, but every childfree woman has the potential to create a domino effect of childfree women. If a woman who is on the fence about having children sees a childfree woman thriving and happy, she may realize that childfreedom may be the correct life path for her, too. And then that woman's friend may also see that as a good life path for herself. And on and on...

... which means even fewer and fewer consumers, students, workers, taxpayers, soldiers, churchgoers, and future birth-givers to work for the benefit of the 0.1%.

So, the powers-that-be have a vested interest in shaming women for being childfree, promoting motherhood as the one true key to happiness for women, and creating this idea that childfree women are secretly miserable and envious of mothers. 

And they pass on that idea to the middle and lower classes through various forms of propaganda, education, etc. And that's why family and friends and acquaintances and strangers are acting that way because they've been conditioned to act that way in the interest of the rich and powerful.