>Why? I guess it's because of a lack of practice – you don't really need to interact with so many different people nowadays and interacting with people who are very different from you is just plain terrifying for many.

It’s not any more or less terrifying than in previous times, but as you wrote “you don’t really need to interact with so many different people”.

I would shorten that to “you don’t need so many people anymore”. Another factor is you can easily find more agree-able people (or bots) to spend time with, such as on this website rather than a neighbor.