Others have pointed out, but I will emphasize, that this runs smack into the local politics of suburbia. Most people don't want to live next to a multi-unit house, and so: 1) the value of homes around a multi-unit house will decrease, causing enmity and dislike between neighbors, and 2) neighbors will respond by forming HOAs that restrict the number of units allowed in a house. My own HOA, for example, completely bans renting out my entire house, much less subdivided portions thereof.