The same goes for men's parts too, by the way. The men of the Yanomamo tribe in south america wore nothing but a cord which held their penis up against their body. If the cord slipped so that their penis fell down they were considered naked.
These are simply cultural normals. I think the only thing wrong with things like hajibs or anything else, is that some people are forced to wear them. Its the force that's oppressive, not the particular outfit. There are also various methods of force. Making something illegal at one end of the spectrum. A society that has a great deal of peer pressure is simply a milder form of force. I recall as a teenager, with respect to swim suits. I was very slender, and I looked good in a bikini. BUT, there were times when I simply wanted to wear something more practical for swimming. My favorite, if peer pressure were not a factor, might have been something like a one piece suit with legs on it, at least an inch or two long (like shorts). But such a thing was considered silly or old looking or whatever. It would have been ever so practical. Wouldn't have to shave so high that one gets a rash, wouldn't have to worry about things riding up, or falling off on top. And yet, I never dared to seek one out (if they even made them) because I was afraid of peer shaming. Sad that we simply can't let people be, and express, whatever they want.
ETA: One more example. A friend of mine was at a retreat where they had a "swimsuit optional" hot tub. Seems that everyone interpreted that as a nude hot tub, so she didn't want to go in. Luckily, a friend told her that swimsuits were fine, even though everyone else was nude. Perceived peer pressure can be so strong, even when its only our own internal voices doing it to us.