It's not the GP being less social - it's their neighbours as well.
They said other people don't knock, which is what make it seem like it would come over as a weird if they did it themselves.
That chimes with my experience. In ~25 years of living in many different places, I don't think I've ever had my door knocked on by neighbours for an introduction or friendly chat. Only by people I already know, and that's rare too. (I don't live near anyone that I know).
So I wouldn't knock on a neighbour's door. I'd feel uncomfortable, intruding on other people's private lives.
It's not like I'm not social. I like people and would probably enjoy knowing my neighbours better. (Though not during this pandemic - I haven't mingled indoors anywhere since March 2020).
But it seems like every little group of people (most are houseshares) keeps to themselves.