BJJ is very social because you can't train alone. It also mostly weeds out assholes because no one wants to train with jerks (usually there's a single gym in an area that ends up with all the jerks lol). The other thing that quickly forms relationships is that you're trusting other people not to hurt you and vice versa. Giving and receiving this trust tends to cause relationships to build quickly.

And yes, many gyms are filled with nerds. Once you get past the basics, it's very much a thinking sport.