We may have a language problem. Here, a "couple" is just a pair of people dating. It carries no formal or legal status.
Some states have a concept of "common law marriage", where the couple has lived and represented themselves as married for long periods of time without actually having signed the legal documents. But the vast majority of "couples" here don't fit that definition. Here, you can be a couple for years, even cohabit for years, and not be legally bound in any real sense.
In the US, marriage is not just a ritual. It includes the signing of a legally binding document which carries with it a number of rights and responsibilities.
Here, it does. Specifically, in this case, the matter of child support is determined as part of divorce proceedings. There are no such proceedings for a couple.
To quibble a little bit on US Marriage:
child support being mandated can happen regardless of marital status. Get a girl pregnant and identified as the father, and you are on the hook for child support.
Though otherwise, you are correct. Marriage entitles you to inheritance should your partner die. As well as insurance benefits (can't list your roommate on your health insurance). It also entitles you to access and information (as the Supreme Court case was about). You can get shut out of the room when your partner goes into the hospital, and under HIPAA you don't get told what's going on. Marriage solves that.