I can run a conversation just fine, thank you very much! (fakes annoyance)
It's just that I don't very often get an overwhelming urge to express myself to others, I am quite content with my own opinions, but don't mind the opinions of those I trust. Of course, that may mean that, in the end, all introversion represents is the egotistical belief that we alone are right, and so right that we don't need the external confirmation of our absolute correctness by being social (either that, or we secretly fear we are wrong and are afraid to find out we are not).
I know many extroverts with whom I can carry on a conversation, but they still wear me out.