People don't want to be around others (the fear aspect), so they drive their own bubble around.
You say this like it's a bad thing. It doesn't have to be fear, it can just be preference. There are introverts and asocial people, neurodivergent people, people with social anxiety, and any number of other reasons that they don't have to justify to you as to why they would prefer not to be around a mass of other people. Social interaction in cities tends to be of an impersonal gesellschaft nature anyway, so it's not like you're missing out on a genuine social experience most of the time. It's fine to just interact with those you personally vet and invest in.
https://en.wikipedia.org/wiki/Sealioning