Not sure what you mean? I didn't say it was always done. In fact the point I was making was that it wasn't always done. I gave my own counter-examples. It's just when people look at modern planes they always see two engines and think that's the way it has to be. It clearly isn't.
The MD-80 (and 727) were old turbojets though. Probably a lot harder to come up with a practical tailmounted design with a turbofan although I'm open to it being possible to mount one turbofan above the fuselage.
The MD-80 had two rear engines and was a fairly popular plane. A lot of smaller jets still have rear engines, too.