I think that there is for sure an aspect of underdeveloped skills. That could come from lack of interest, background/culture/environment, or any other number of reasons sure. But I think a bigger aspect is how psychologists often phrase it (sure it is wishy washy) but where do you get your energy. Some people are going to be more comfortable with people around and others feel more relaxed alone. That can and most likely will change over any individuals life but it is important to recognize what environments to go to when you need a recharge. And yes, people love to self-diagnose and take surface level quizzes, but this also does seem to be a real phenomenon different personalities have and understanding your own and others personalities will improve everyone's social interactions. So I agree, we should all work on being with ourselves as well as how to interact with others. But we also need to know, when feeling stressed, if we should retreat to a quiet room or a crowed get together.

And just a last point is that it seems like this silent book club is doing what you suggest, it is an introvert going to a place they feel more comfortable which will in time develop their social skills.