Do people really only know people who do what they do? Of course you expect a coder to know more coders than the normal population, and the same for doctors or any other profession.
But do you not have friends and relatives from all three of these categories? I don't even live in the Bay Area, but I have Bay Area friends and relatives in all three.
Is your standard for friendship facebook friends, of which the average person has hundreds, or close personal friends of which the average person has 3-7?
Did they grow up where they're living, thus forming a non-work social network, or did they move there for work and find the distance made it hard to keep in touch with old friends?
Do they have interests (religion, sport clubs, parenting etc) that bring them into contact with people outside of their professional circles, and give them a chance to get to know each other?
But do you not have friends and relatives from all three of these categories? I don't even live in the Bay Area, but I have Bay Area friends and relatives in all three.