The entire peninsula is unaffordable for low-income workers. Crossing the Dumbarton bridge is a ~30m commute in itself and costs $5/day. Downtown Palo Alto just instituted parking restrictions on the surrounding neighborhoods so low income commuters no longer even have a place to park.
To clean a house in the area can cost ~$150. Good nannies can make $50-60K a year. Plumbers are expensive. Gardeners are expensive. And so on. This is just basic economics.
Also, I think the commutes are somewhat blown-up. Plenty of cheap places in San Jose that aren't much more than 40 minutes away if you avoid rush hour.
Leaving aside the question of whether or not 40 min is a reasonable commute, you don't get to determine if you avoid rush hour, and you probobly won't. So 40 min is at best a lower bound rather than what you would expect to see.