"You are not alone
We are here with you
Though we're all far apart
Seems we all have the same heart"
I'm also over 40 and single - no kids, never had a girlfriend, no anything.
"Why?"
Mostly from apathy. I never change something until there's a reason to change. I'm a nerd but I never run out and get the latest Windows just because it has been released. I always upgrade ONLY for a reason. As my life is running fine right now, I see no reason to change it.
"But... don't you ever want to get married?"
No. I don't know anybody I want to marry, and I see no reason to go out looking just for the sake of checking "got married" in my list of life's accomplishments.
Now if I happen to meet a nice lady, and if we happen to hit it off, and if we start spending a lot of time together... then my answer may change. I might at that point want to get married. But getting married just for the sake of being married? No thank you. I'm not so afraid of being alone that I'll run out and actively look for somebody who is willing to be with me.
I'm not exactly working hard to carry on that family tradition because as Lynx says, single and childless is literally the default option, it doesn't take any work to be single.
I can't take the credit. The phrase came from an xkcd comic about a guy who has never seen Star Wars.
"But... How?!"
"Uh, it was easy? It was literally the default option."