I'd argue that the 'gang' at work serving as a social network is a big part of why people end up isolated. Because there's usually no compelling reason to socialize with work people outside of work. So if you're not particularly social anyway, once work ends your social network evaporates.
I will say that the envy/fear complex around groups of people socializing and having fun cuts a little close though.
I wish I could tell his that this gets better with some more social contact, but ultimately being alone and old just puts it into starker contrast with the creeping realization that everyone else has something meaningful to go home to and you don't. All the parties don't change that.