Or, "You're so lucky to get to stay at home all day." Free time? Are you kidding me? Sure, my schedule isn't dependent on office hours or a boss's needs. But my hours are dependent on that little human being who wants to show me the block tower she made while I'm trying to clean the bathroom. Or she insists on me reading book after book over and over again instead of preparing for dinner.
There are also errands to be run, diapers to be changed, laundry to be washed, beds to be made, babies to entertain and outdoor time to be had (and I do not mean much-needed yard work). Sure, I can see where people might assume that the days are open for flexibility. But as a SAHM, you're a slave to nap time, lunch time, snack time, diaper time, playtime and give-me-more-attention time ... while also trying to focus on that role of homemaker (cooking and cleaning and other need-to-be-done tasks) that seems to go part and parcel with the role of SAHM.