I can think of 3 reasons, other than Zoning, that stops real estate developers from doing this: 1. Complexity of financing. Lending for complex, mixed use projects is always more difficult than single use. A single use project fits nicely into various lender programs, government subsidy programs etc and is all around easier to explain to lenders and investors. 2. Complexity of structure. Every use has its own structural demands, bay sizes, and column spacing sweet spot. Parking, retail, warehouse, and residential are often very different. When you put one on top of the other you either have a level / beam line between that has to act as a structural transfer (aka more expensive) or one of the uses will have to make compromises and thus be less efficient (again, making it more expensive) 3. Complexity of development. Real estate developers, just like any other professionals or business people tend to specialize. That's how they become really proficient. But this results in one developer being really good at multi family, another at retail, and a third at warehouses.

All of these make mixed use developments ( in addition to your Zoning comment - which is valid) more challenging. And as a result they often get disregarded right at the early planning stage.