Of course, one could write a whole article on that alone.

I might even do that.

The North American landscape is dominated by single-use zones: massive slabs of land dedicated to one use only, such as housing districts, financial districts and malls. Instead of having your work and shopping around the corner, you're basically forced into a car to get where you need to go on a daily basis. Lewis Mumford wrote about this in 1961, warning us for the future:

The end product is an encapsulated life, spent more and more either in a motor car or within the cabin of darkness before a television set. [...] Those who accept this existence might as well be encased in a rocket hurtling through space, so narrow are their choices, so limited and deficient their permitted responses. Here indeed we find 'The Lonely Crowd.'

While the full text is more doomful and dystopian, there's definitely truth in it. But you can't entirely blame the city for that. It has just as much to do with our increasing individualism, the decline of the communities and even the increased online contact. We seem to forget that to meet new people, we need to step out of the seemingly solid community circles we live in. Try something new once in a while. There are tons of small communities where you can meet lots of new people, it's just that you need to go look for them.