Because sometimes there seems to be little choice....for example, my lovely SIL wants to be a SAHM, but has to work because my brother is a fisherman and, especially this year and now with his boat sunk, without her income they wouldn't be able to pay the bills.
My BF wants to return to work, but can't because she can't earn enough to cover childcare costs, even though her DH works FT.
Another friend is often envied because she has a sports car, a high paid job and several holidays each year - but she would give it all up for a baby and a partner who wasn't an arsehole like her exH.
It seems that few of us actually seem to have much choice in our lifestyles these days.
Or maybe I've just got too much time on my hands (recovering from a knee op this week and bored and childless as they are away camping with DH).