False analogy. To marry, you don't need to be qualified for public safety.
Even if you proved competency in safe driving, that's not all drivers licenses dictate, or what is involved in the relationship between government and citizen license (as well as ID laws). There is no separate contract that is just safe driving competency.
You can drive without a license, just not on government roads. You can't get federal government benefits relating to spousal merging of holdings without a marriage license. (Domestic partnerships and civil unions are state level contracts, not federal).
You have the domestic partnership laws plus a domestic partnership agreement which, if properly drawn, should take care of the legal issues.
Making separate but equal contracts
is unconstitutional. We already have a contract covering legal issues. It's called a marriage contract. Domestic contracts are something different, providing specific state level and some federal level benefits.
Why isn't it something the government can just assume unless directed otherwise by the parties? It's a simple matter of writing it into the law, isn't it?
Nothing about lawmaking is simple. And I already said why they wouldn't assume it: inheritance requires consent of the party to merge wealth. Living with a someone, even a lifelong partner, is not consent to inheritance. It would fall to next of kin without a specific POA, living will, or marriage contract which dictates otherwise. Marriage means the partners have stated that their assets are merged, so a partner automatically receives the others property upon death unless stated otherwise by aforementioned POA or living will.