The school may dictate the time so staff can attend.

At our Big-3, lots of moms are both older and have established high status careers. This affords them flexibility so they show up with somewhat surprising frequency to school events. These career moms are not necessarily working for the income, as many of them appear to be married to people who are also successful career-wise. It's not about the money at this point.
The SAHMs mostly seem to have really little kids at home (and eventually end up back in the workforce). I really know very few SAHMs with kids in the highschool; most of the moms who were SAH eventually ended up back at work and the ones who didn't had extenuating circumstances (health issues, kids with disabilities, etc.).