Introversion/extroversion is a spectrum, and if you put down a line in the middle, I’m sure you’ll find a significant part of the population on the more introverted end of the spectrum. We don’t notice because of selection bias: the loudest are most easily heard, tautologically, and so we notice those whom we’d be expected to notice (those making themselves heard, i.e., extroverts) while those we don’t notice are those we wouldn’t expect to notice (the quiet ones).

You’re right that the normative behavior, the behavior that is expected and promoted by society, is extroverted behavior. But I wouldn’t concede that introversion is odd. That would be like saying homosexuality is odd. Homosexuality isn’t odd, not even technically odd. It’s not unusual. In this day and age, it’s odd to regard it as odd. I’m sure there are as many, if not more introverts than there are homosexuals, and I don’t think we should regard either as unusual, odd, strange, weird, non-normal, etc.

I understand that you can use the word odd in the way it would be odd to meet a guy who could shoot milk out of his eyes or benchpress 300 kg: as something not necessarily wrong or creepy, but certainly out of the ordinary.

And I don’t really see introversion as anything unusual.