Because the world doesn't work like that? if it did, we would be paying the same proportional amount of income to rent today in 2016 as we did during 1970. We don't. Just because there is more money doesn't mean that you get proportional increase in construction, maintained demand for housing, or demand for land for which houses are built on. It doesn't change how government rezones in order to control rents in cities.

Rent is not some basic formula derived from the average income.