This seems to be very true. I was the first to move in on my block. Used to know almost everyone --- we even had seasonal block parties. I think that I'm the last of the originals and I know almost no one. Most won't even look up to wave or even just acknowledge you. It kinda feels weird to be alone in the midst of so many people.
The theory is, it's a lack of front porches. People live outdoors in the backyard now. We spend most of our time engaging people, the last thing we want to do is engage the people next door. Be the neighbor, make cookies, ring a doorbell and introduce yourself. Also, don't talk politics or religion. That stuff belongs in your underwear drawer.