I have to suspect some of that is your perception of it, and in reality, many of those 99.99% were not quite so happy and content with their social lives. From my own experience it seems being autistic isn't having a monopoly of misery and isolation, it's just one route to it.
As best I can make out, many apparently normal people often see everyone else as being involved and included and happy, when they feel disenfranchised too, and often keep up an act (a sort of masking?) of their own as much as many of us may do.
I think the symptoms are more common than it may seem, but have many different causes. It's less the actual activities that are being missed out on, but rather missing the inclusion of being in a like minded group. If all in the group are into playing D&D online together without even meeting in person, I'd say that's as meaningful and important as going out boozing and clubbing etc. (one night stands aside of course!
)
(actually, I'd find it more meaningful than reducing myself to a sweating boozy vomiting mess by the end of the evening!
)
Many go out to do the party thing because it's what's expected, the peer pressures, not because it's their most sought after activity. It's the socially accepted way to find potential partners, to establish within a sub-culture, it's an acceptable way to be pretend rebellious, it's yet another form of social hierarchy juggling - each finding their 'level' in society, who gets to dominate, who gets dominated, and so on.
The more I saw it for what it was (from my perspective) the less I wanted anything to do with it. I used to join in when young because I didn't know what else there was, and didn't want to lose a small group of friends, but never for the pleasure of it.