I'd like something between hooking up with a stranger and recreating my life (again) around a new person. It's hard to do that once you're older and I'm actively avoiding being financially intertwined with another person.
Besides having a lot to lose, I also question if I'm desired because of what I can financially bring to the table and as a labor source. Maybe I'm cynical, but this seems to be the calculus of a lot of single parents. I'm not looking to be the solution to someone else's life problems.
Many people have an expectation that boyfriend/girlfriend status is essentially a trial period for marriage. If it's not "going anywhere" it's a waste of time.
As people age, if they have this perspective, they tend to become significantly less patient.
if raising kids isn't a factor, I bet many couples would be happier living on both sides of a duplex.