I dont know about you but i remember a few games that have done this. Lets see, Diablo II, Diablo 3, Diablo 3 again (Monster Power), World of warcraft (come on man what do you think the difference between normal and heroic dungeons or normal and heroic raids are? You up the %'s and add 1 or two mechanics if that.)
Lots of games do this cuz it works. If a game is easy cuz u can kill mobs fast and they dont hit you that hard then all you have to do is make it so you cant kill them fast and they hit harder. They do it cuz it works.