Seriously, why don't they? I believe having this be the mechanic would have several positive influences on the game:
a) It would keep old content relevant and prevent its obsolescence.
You could potentially visit any dungeon in the world of previous expansions and still find it challenging, rewarding and worthwhile provided of course your damage was scaled down. This is better for new players in my opinion because it gives them a wealthy pool of content to do and experience.
People have said and indeed Blizzard themselves have said that they want us doing the current content. Why exactly? There are so many dungeons and raids out there that are not being used at the moment and I believe that is a big shame. Also the current content is miniscule in comparison to all dungeon/raid content that exists out there.
Then there is the argument by Blizzard that creating hard content that will be experienced by only a small portion of the player base is financially unviable. I find this interesting, especially seeing how 90% of the dungeon and raid content of the game is ignored now.
b) It would stop gear from being a factor determining your chances of success when attempting a boss
and would instead make the fight entirely skill based so you couldn't use lack of time as an excuse to do a raid any more. Granted, this would apply more to smaller raids, not 14 boss ones.
c) It would obviate the need to have multiple difficulty levels of the same dungeon.
This is due to the fact that the challenge presented by the fight would be equal for everybody, regardless of your gear (i.e. health pool), since 20% of 20.000 or 300.000 is the same in relative terms - you still lose 1/5 of your health. I find the current model a bit overblown because you do the same dungeon ad nauseam due to all the different difficulty settings since gear has such an impact on your performance in the higher levels of difficulty. By the end of it all, you're bored stiff.