I add some qualifications. First, people outside the academy have time pressures of their own, which are different from the time pressures on people inside. (That's a frequent topic at 11-D.) Second, it's not unknown for college graduates who hire out with the private sector to uproot themselves (also true of MBAs and lawyers.) It is true, however, that a lot of the work of the academy, particularly the stuff for which one earns one's pay, such as grading or re-checking calculations, is best done in quiet with the door closed.
In part, we have to isolate ourselves in order to do our jobs. Grading is not social. Research is not social. Service commitments infringe on our weekends and evenings. In choosing this profession, we choose a lifestyle. But whereas making this choice may not seem very dangerous to us in graduate school, when we have a cohort of people to hang out with and who understand our experiences, or when we begin to think about joining this profession, when we probably still live in or near our hometowns, and have strong networks of family and/or friends who are geographically convenient to us, the reality is that most of us don't get jobs in our hometowns or in our grad school cities. We get jobs in places where we probably won't know more than one or two people if we're lucky, and we need to start from scratch. (And even if you did get a job in your grad school city, you're still weirdly isolated because your friends leave, or those who stay are still in a different life place from the one you're in, or whatever.)
The problem is, the vast majority of grown-up people don't start from scratch with making friends. They just don't. At a certain point, as Oso Raro noted in the comments over at New Kid's, friendships happen more organically. It's not like being 14 on the first day of school and having an expectation of making new friends. For grown-ups, the expectation is that you've got the friends you've got, and while you may meet new people, you most likely will not become "best friends forever" with most of them. But as an academic, uprooted from the social networks one builds over a lifetime when people ARE making their best friends forever - from ages 0-25 or so - what is a person supposed to do? Because when you move to a place, you NEED to make BFFs, whether you're married or single or somewhere in between. Because people need friends.
BEFORE YOU GO FOR THE PH.D. I'm still catching up on some posts I flagged for further reference. This Reassigned Time post appeared early in September, but with the GRE scores and letters of recommendation going in the mail, it's useful for the latest crop of novices and postulants to understand the nature of the work.