Why is menstruation still treated like a secret mission?

TL;DR:

My mom once waited the whole day for my dad to buy her pads, even though I was home and could have helped. My sister, like my mother now, hides menstruation-related things as if they’re shameful or secretive. I find this stigma around periods absurd and wish it could be normalized within families.

I’m a 19-year-old guy, and I’ve noticed a pattern in how some women around me treat menstruation. It’s like it’s some top-secret mission. My mom has always tried to hide the fact that she’s on her period. For instance, she stops going to the puja room during that time—something I find absurd. If God created women this way, why would he suddenly stop being okay with it? She also hides sanitary pads like they’re classified documents. Once, she waited the entire day for my dad to buy her pads instead of asking me, even though I was home the whole time.

What really struck me, though, is how this behavior seems to be passed down. My younger sister was once open about her periods. I was actually the first person she told when she started. But recently, she’s started acting just like my mom.

Yesterday, mom brought groceries home, and when I went to help her unload the stuff from the scooty, she deliberately left the pads behind. She told me to take the rest of the groceries inside so she could secretly grab the pads later, as if they were nuclear codes.

This secrecy baffles me. I mean, I’m 19—it’s not like I’m unaware of menstruation. It’s a normal biological process! Why all the shame and secrecy?

I’ve thought about confronting my mom about this, but I don’t think it would change anything. This behavior seems deeply ingrained, probably from how my mom treats it. But I can’t help wondering: why is this still a thing? Why can’t we just normalize it?

What are your thoughts? Have you experienced something similar in your family or society?