Dear Boy....
The simple reason is that one only finds feet attached to the lower region of one's ankles. Height (or altitude) as measured by the terminally earthbound, such as air traffic controllers, may refer to one's height (or altitude) in feet, but one would never refer to a boat for instance as 45 feet long. Feet are exclusively attached to ankles, foot is a mearsurement of length (as in some appendages) or height, so in my humble "olde english" experience, feet in aviation terms should be incorrect.