The concept that married people live longer is interesting. I'm sure there is some merit to the idea that if you're married there is someone there to nag you about going to the doctor, but I think much larger factors are having the finances of dual incomes and access to an immediate support person.
Surgeries require having a designated person to look after you. Many injuries require driving to somewhere like an emergency room which can be hard to do if you are the one injured. If you're home with the flu, it's hard to tell when it's bad enough to go to the hospital without another person checking on you. And if you pass out it requires another person to find you like that to get medical aid.
You can prop it up as the benefits of marriage, but I think there's a much deeper discussion to be had about how we've built society around marriage as an inevitable conclusion and neglected to build support systems that function outside of romantic pairings.
thinking about this further, people often cite this as a sort of See It's Better To Be Married and mostly accept it as a fact that being married is better for you overall and proceed to breakdown why marriage leads to longer lives. Instead I think we need to be looking at why the system is failing single people and what we could do to close that gap. What structural societal changes can we make to help single people rather than treat it as a Well Obviously foregone conclusion that everyone will eventually pair up.
It's not Why Are Married People Healthier? It's Why Aren't Single People as Healthy? And then actually examine the causes rather than hand waving it away with whichever stereotype of being single or half remembered memory of the last time you were single in your early 20s.
