The sad truth is the lack of paid maternity leave is just a symptom of a larger women's rights issue happening in the US. It's the same reason we're still talking about equal pay for equal work and abortion rights. President Obama said it best when he said "If men had babies, we'd have different policies." Sad but true.
Some companies, cities & states do have their own policies, there's just nothing on the federal level. Like I know Facebook now offers 4 months of paid baby leave to all parents but that's pretty rare.