Because the cost of pregnancy and newborn care needs to be spread across society — it’s not possible or fair for women of child-bearing age to bear those costs alone. You benefit from healthcare provided to moms and babies (you were born, we all were born, the women in your life may have had children, or may someday). If only women who intend to have children buy maternity coverage, providing that coverage becomes extremely expensive extremely quickly. People are already delaying children these days, so if having an extra tens of thousands of dollars saved to cover healthcare costs becomes a requirement (not for the baby-before the baby is even born!), the birthrate will drop even further (bad for the economy and older generations looking to retire). And don’t think “Oh, responsible people save up for that.” No one can save for the cost of an antepartum hospital stay of there are complications (can be weeks, or months!) Or a NICU stay. Also, what about people who want to become pregnant, buy incredibly expensive pregnancy coverage, pay for everyone else’s maternity care and then experience infertility? All that money and all that stress and no baby? That seems injust to me.
I didn’t mean for this to turn in to a rant, but I really can’t wrap my head around it. “I’M not pregnant, I won’t pay for it/You’re the one who got pregnant, you pay for it” is just not an attitude that makes sense in this situation, and actively harms the women and children in your life.