Some people are smart enough to know that they have a problem in social situations. However, those same people are usually not smart enough to fix the problem. They wonder why they don’t have very many friends, but when the opportunity to make new friends or maintain old friendships poses itself, they go running for the bushes.
Don’t ask why that is; it’s just the rules of nature. It’d be like asking why lions eat gazelles or why cold water takes spiky while warm water tastes smooth. So, if you see an introvert in the wild, don’t approach them. Odds are, they’re doing their best to hide from you.