here's a few reasons.
I am there to help with lunch at least once a week. I go on every field trip for which drivers are needed. I pitch in when something's needed as I am in and out of the building.
I consider myself a SAHM, but until recently I worked a part time job a couple evenings a week - as do many - so all those evening meetings were just out of the question. Besides that, SAHM evenings are just as packed as working mom evenings - dance, music lessons, sports, Bible study...its in the evening for us too, even if we are home in the day.
Even if that weren't the case, I'm just not a meeting kind of person. We have this same thing at church. There are people at the meetings planning big a-few-times-a-year events, and those of us that do the less spectacular work week after week. Like Sunday School, and delivering things collected in drives, and helping with youth group, etc etc
When my oldest started school, I signed up for room mother 2 years in a row. It then came to my attention that people who needed very scheduled occurences that they could plan half a day off for were wishing for those spots. So I stopped signing up to be in charge.
Thank God for making us all different! There's jobs to suit everybody and our kids need us all. Between us, everything is covered,

If you meeting types didn't cover the big events, that would be a loss, and if I wasn't there for mundane day-to-day stuff, that would be a lss as well.