I'm not surprised that so many posters report that their in-laws have never met each other. That's to be expected when young people move away from home, but our experience has been different (not necessarily better or more desirable, just different).
We met our oldest son's girlfriend's parents when they helped her drive from their home in St. Louis to Philadelphia, where she and our son were in medical school. They'd been dating for about a year at that point and have now been dating for nearly four years. They're lovely people and since then we've hosted them for Thanksgiving a couple of times. Last month we met up for dinner in LA because we'd discovered that both sets of parents were going to be in LA for completely unrelated events.
We met our middle son's girlfriend's parents on that same trip to LA. They both live there and decided it would be a good time for us to meet. They've been dating for about a year now. Parents are also really nice and fun. DS commented later that he was worried about how the conversation would go, but found that asking us all to recount our experiences in the 1989 earthquake was a great ice breaker.
We met our daughter's boyfriend's parents at dorm move-in when they were first-year college students. Of course, at that time, we had no idea they would even become friends. They've now been dating for two years. Again, we're lucky -- the parents are very nice people whom we've seen here and there over the past couple of years at college events and move-in/move-out. When we drove to Maine last summer, we also stopped to have lunch with them near their home.