It's an interesting question why marriage is able to create that first effect, and I don't have a good answer.
I can think this is because it is an act that is supposed to entail the following:
To me, those things are implied by the "non-betrayal" stance. Agreement on childbearing, shared financial interest, and pair bonding (i.e. shared emotional interest) are consequences of the fundamental agreement not to betray. As you note, each of those could be achieved without marriage - but most people act as if this were not possible. I'm just as confused as you.
That is different from noting the incidental benefits of legal marriage - if I die without a will, my wife gets my property. To achieve the same effect without marriage, I'd have ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.