Why do you say this is typical? It depends on what you rent, of course. If you rent a house that was purchased a long time ago, it can be less. But a lot of rentals are in apartment complexes, and are newer and more expensive per square foot than equivalent purchases. I also think typically rental units are smaller than houses for most people, so I wonder whether you're really comparing apples to apples. Consider that for any given house, rent must cover the owner's mortgage, so it cannot be less than the purchase costs.
> if you're renting you have an extra $X/mo to invest in the stock market.
People usually use the down payment as the capital opportunity cost that you can invest as an alternative to buying a house. My question is this: statistically, how many people actually do it and make better returns than buying a house? It's one thing to talk about it being possible, and another to actually do it. I don't know how to consistently make better returns investing than I've made on my houses, and I'm not sure I have the energy to stay on top of it every month. Note your suggestion to invest instead of rent involves monthly adjustments of your investments.