It’s not a conscious decision that we make to “hide” it each month.
The practice and purpose most likely evolved around cultural practices beginning with those that ostracized women during menstruation, and then those that deemed sex ‘dirty’. We reach an age of fertility long before we are culturally ready to procreate, so the adolescent females not ready to marry are not supposed to advertise that their eggs are ripe. As society has become more complex, there are things that need to be tended to. I can’t take a week out of every four off because there’s blood pouring out of my crotch.
Besides, with humans, ovulation can even occur during menses, so there really isn’t a way to know for sure when we are actually fertile. Some are more regular than others, and I have found that the signs were more obvious the older I’ve gotten, but I couldn’t really say with certainty whether it is because I know my body better, or whether it’s because my body is changing. Most likely, it’s a bit of both.
As we’ve developed culturally, we also aren’t driven by the need to propagate the species the way animals are. Our society expects us to be responsible about when and with whom we reproduce. Some cultures allow couples only one or two children; very few modern cultures support the idea of having a dozen or more children. It’s a product of human civilization.