That math is a bit funny. Rent is pure consumption while mortgage is consumption plus equity accumulation. Of course, owners have less left for investment, but they are already investing a significant part of their mortgage payment. With 15 years left on the mortgage at a 4% interest rate, about 60% of your payment goes to principal and only 40% to interest.

And since most people prefer the flexibility of rent to the headaches of ownership, rents tend to be significantly higher than the non-equity part of the mortgage, so ownership is a more efficient way to invest and live at the same time, precisely at the cost of that flexibility of consumption.