Married people report significantly higher levels of happiness/life satisfaction in the middle and later stages of life than non married people. This is a durable finding across many studies.
In the cases of "never married", their happiness level approached that of married people in the last 10-15 years. Whereas divorced people were absolutely miserable, especially in the first few years following the separation.
This suggests that it's the divorcees who bring down the average happiness level of the "single" group, not that "marriage" brings up the happiness levels.
Presumably Married people at that stage are there because they work together not because there's just someone else there.
It could also be that people who are generally happier tend to be those to get/stay married.
I also wonder if there is some sunk cost psychology involved when surveying those later in life.
It would potentially be also interesting to control for sexual activity? Presumably most married couples were sexually active together at some point, which I'd assume generally increases happiness.
As for benefits that don't matter so much I count things like societal approval, "doing what my parents always wanted", etc. Which matter for some, but certainly not for all.
But the wet roads didn't cause the rain.
How many people are going to choose to have a mid/late-life crisis because a survey made them wonder if they were truly happy with their huge, life-changing and nearly irreversible decision to be married?