Because some people are just naturally introverted. Large social situations exhaust me. I need time alone or with 1 or 2 people I known to recharge. Especially after work. I don't want to go anywhere where I have to be social after work. I also don't do small talk all that well and don't have many shared interests with people. It's just how some people are. Many great minds throughout history were introverted. It affords one time to ponder life. To think about and question things.