There are a couple of reasons MDF is unsuitable for this application. It is much denser on the manufactured surfaces than in its core. It is not nearly as stiff as real wood, or even plywood or OSB. I guess this is due to what has been done to the wood fibers in the first stages of manufacturing. There is not a lot of glue in the center. Most of it is on the surfaces. It swells quickly and drastically when it gets wet. It expands and disintegrates when you drill holes or drive screws into it.