Generally, because the character traits that lead people to be good marital partners are also the character traits that lead to financial success. Remember, marriage is entirely optional now. BITD, if you got pregnant/got someone pregnant, you married, whether you were actually prepared to be a husband/wife or not. And if you were 20-something and female and not married, there was something wrong with you; marriage is just What You Did. That's not the world we live in any more. Now, you have to consciously choose that you want to spend the rest of your life with this person -- and then put in the daily work to make that work. And the people who tend to make those decisions are reliable, conscientious, intelligent, physically healthy, mentally competent, not addicted, gainfully employed, able to delay gratification, etc. It's the same character traits that tend to get people through college, get decent jobs, keep those jobs, and be able to maintain good relationships with friends and family. Assortive mating is a real thing.*
Obviously it's not binary; there are many, many people who don't marry who have those traits, and many people who do marry who do not, and then of course there are people who change from one category to another over their lifespan. And some people without those traits will succeed financially, and some who have them will not. But put two people with those traits together, and you have an optimal foundation for financial success.
Why is the gap so huge? Power of compounding, for one. If you have two 401(k)s instead of one, you are going to end up with double the money. But beyond that, you have more disposable income. Married couples with two incomes can pay more for a house, but it usually won't be 2x what their single counterparts pay. Even if it's 50% more than what each paid individually, that still leaves you with more money to save. Add on to that that married people are more likely to buy homes, and when you buy a home your payments generally stay the same, whereas rent will increase over time, and that difference just compounds every year.
*You can make a similar argument with tithing. You'd expect that people who started off giving away 10% of their income would be in the hole as compared to people who don't. Yet everything I've read suggests that people who tithe on the whole do better financially than those who don't. Why? It's totally illogical if you just look at the math. But people who tithe tend to be responsible, diligent, decent at planning, etc. -- which are exactly the same kind of character traits that tend to lead to financial success.
