Yeah, my first thought was, because we're animals that evolved to be quite social and have developed cognitive biases in light of that fact! Bet you can find powerful "groupthink" in introverts as well. . . like, say, an insistence on thinking in terms of introverts vs. extroverts.
A fun article by Alan Jacobs. Check out the paper he cites, if anyone finds an non-paywalled version, I'll edit in the link here. HT for the link to Michael Bloom.
I really do need to find more written by this author. But while I certainly do very much share this sentiment I have a hard time figuring out how common it is. After all people don't look good saying they "don't like meeting new people".