They aren't the only ones. People in this country don't always brush or use toothpaste either. You asked. And it isn't just those who "can't afford it". Many have different methods of caring for their teeth, many of which do not include toothpaste.
The point is that you can go without toothpaste or even brushing your teeth and still maintain healthy teeth and gums using other methods that do not cause some great strain on your life. And yes, you can go without a tampon or pad but it would greatly affect how you lived your life. Something would have to be used or it would cause sanitation issues to all of society, especially if the women simply decided to go out in public and "**** it". Those same people who can't afford to brush their teeth also generally can't afford food, clean water, electricity, sunscreen, medicine, medical devices, candy, and other things that are tax exempt by various states in this country either, yet those things are tax exempt by many states with the reasons being that they are "necessities".
However, to the point of the thread, groups and legislators are asking for something that women in this country do need on regular basis, is taxed, but other things that are comparable get an exemption from such taxation in some states due to their being considered a necessity but this doesn't. They want that to change and are pushing for that change.