Because every couple is different. Marriage is incredibly simple. The actual complexities of marriage don't come from marriage itself but from all the shit left undefined, which has to worked out ad-hoc per couple.

I'm sure if you were the dictator and God-King of a small country, where you could reinvent society as you wish, you might be able to create something that would fit 99% of the population, since you would be able to forcibly squeeze the behavior of each couple into a few standardized containers with corresponding legal rulesets.