I think a lot of these "hobbyless" people actually just do typical things we wouldn't consider a hobby, and that takes up enough of their time to sufficiently get away from each other in relationships. I.e. television, cleaning, cooking, grocery shopping etc.
And they're probably better looking than us, that's why they never developed anything more interesting at a younger age. Socializing with other good looking people was a hobby in itself.
I likewise have a theory that as single lifelong good looking people navigate the dating pool in their 30s and 40s, they realize the lessening value of their looks. And in turn, they make an effort to engage in new hobbies and be more interesting. Since they already have the advantage of (generally) fine tuned social skills, they flourish in these hobbies by networking and joining clubs and finding others to learn from, etc.