Fifteen years ago I could see the value in tower cases for PCs but I don't understand why people still use them. Your motherboard has almost everything built in, the only expansion card you need is for video and perhaps not even that. The hard disk will be an SSD (unless you are really strapped for cash - in which case why are you building a system from scratch instead of buying secondhand?). There's no need for a floppy drive so all you need is a single drive bay for a DVD player. Why have all that empty air inside your computer?
I know that mini-ITX motherboards and cases are available but why not have a smaller case for an ATX motherboard?