The problem with marriages is that it depends too much on case law, which is, as
@TLK Valentine said, reactive instead of proactive.
People see all the terrible court drama it takes to manage marriage problems, disputes, and dissolutions and fail to realize all of that is because marriages start out on nothing but implied and traditional expectations of performance for both parties. They almost never have the same picture in mind of how the marriage will function. Even if they've been living together for years, one party will have expectations for a change in the relationship--or no change in the relationship--that the other party doesn't have.
The reason there is so much case law is because because expectations were not hammered out in the beginning. That's what a domestic partnership contract would resolve.
And as I said before, it wouldn't take very long at all for the legal industry to put the most usual and common expectations--as already determined by existing case law--into boilerplate. It could even be done on LegalZoom, just as wills are done.