Armchair hypothesis: these careers are highly correlated to extro- and introversion. Introverts are content in their relationships and/or less frequently get themselves into situations that lead to divorce. Call it the nerd coefficient.
There are very few jobs where simply "doing your job" gives you this opportunity.
Also, if you're willing to suffer through difficult higher education, imagine the pains you'll endure in relationships to make that work also :)
This reminds me of a quote from Paul Grahams' "How to do What you Love" (http://www.paulgraham.com/love.html): "If you think something's supposed to hurt, you're less likely to notice if you're doing it wrong. That about sums up my experience of graduate school."
I don't know many of my jock friends growing up that had a "girlfriend" in an MMO, as an example.
You're right to distinguish shyness from extroversion; as someone who became less shy without becoming any less introverted, it's not a clear association.