Why? Just because they are necessary doesn't mean you get to have them just by existing. (Though I think except in rare cases only tampons count.)
Tampons were invented during WWII to help stabilize gunshot wounds so soldiers could get transported to hospitals without bleeding out. Red Cross nurses figured out another use for them and voila.
Why is is such a bizarre concept for people to have access to free healthcare?? Yes, this counts as healthcare. Women did not choose this part of biology, we can't stop it, so why should we have to pay through the nose for products to help us stay clean and healthy? Did you know that menstruation blood can actually kill a woman if not properly taken care of? Probably not. This is a serious issue that needs to be fixed. Having access to free products would make the world a cleaner and healthier place.
If men had something similar, I can tell you with 100% certainty that the corresponding products would be free. Without a doubt.