I do believe Introverts have a higher collective income, too(even in spite of the INFPs).
For this reason, companies that get into typology tend to encourage their sales people to learn what appeals to Introverts. It's pretty hilarious stuff to read about, actually.
Perhaps because they're likely to take longer to decide on something, and when they do, they stick it out. Es would move once they lose their motivation - given this motivation is external I'd surmise environmental changes (e.g. colleagues leaving) would be a greater push for Es to want to leave as well.
I'd have thought ENTJs and ESTJs would earn more though. *wonders*