And why do they have only one kid? Since they have been married for ten years they have certainly had time to have another one.
Well, my parents were married for much longer than ten years; they love me to bits, and apparently actually *wanted* more children, but all they have is me. And, of course, I'm not the only only-child in existence. Sometimes things just work out that way. (Although, to jump ahead, I was hoping seasons 3 & 4 could deal with a second pregnancy *g*)
It's also possible that they couldn't afford another one.
