If I'm understanding your question, it's really the same problem for WOHM's. I work at home and dh works outside the home but my son asks us both "Why do you have to work?" or "Why do you have to go to work?" The first time he asked my dh said, "Because I have to make money." I would always throw in "And because we like to work" and try to explain all the things we needed the money for. I've never been 100% happy with this response but I don't know why. It's truthful, although I wouldn't be surprised if ds thinks we literally make money all day, like stamp out coins and print dollar bills.
I am really interested to see what others say. I've almost posted this question myself (if I'm understanding your question).

I am really interested to see what others say. I've almost posted this question myself (if I'm understanding your question).