Sometimes people don't understand an introvert's need for quiet. And believe me, if you ask any introvert, they will answer that quiet is a needed part of life.
The reason for this is because introverts need to process internally a lot more than extroverts do. It can take a lot more time to work out what is happening, to decide what we want to say, and then get up the courage to act on those feelings.
But just because I'm quiet doesn't mean I'm not taking everything in.
Introverts do care. They are paying attention. They may just be choosing not to vocalize what they are thinking.