What percentage of home owners are people who have owned their house for a long time (to benefit from prop 13), need zero work done (unlikely if you've owned your house for a long time), and whose behavior can be reduced to a financial equation?
My guess is that a lot of people do interior work without permits. If you're tearing down an interior wall and re-building a bathroom, there's no evidence of that from outside, so no simple way for the city to stop you.
There is tons of this in SF, notably the huge number of illegally built "in-law apartments," where someone converts their garage or basement into a seperate dwelling unit without permits.
