Maybe SAHM's stay home because they have a social anxiety disorder, and it is more comfortable to be at home with children than it is to be in a working environment.
I am saying this as a SAHM who has children in middle--high school that are perfectly capable of being alone for a couple hours every day. I am a SAHM who spends M-F with no kids for nine hours everyday, plenty of time to have a job.
I have a degree, skills,more than twenty years of solid work experience, references--I could have a job, but I choose to stay at home so that I don't have to hang out with strangers.
I am a SAHM and the number one reason why I am a SAHM is because I decided to take a break from humanity.
I chose to be a hermit--and because I have kids SAHM was the perfect excuse.