It's to redress the problem with Morrowind,, where you could get to the point where you were godlike in power compared to the rest of the world. Remember, they introduced Oblivion's infamous scaling to counter complaints of lack of challenge for higher level characters. Now the game is built for characters, and enemies, up to level 50, you should always find a challenge. Going above that will take a long time, all your skills being higher so increasing slower, and the game won't throw enemies at you that require levels over 50, or a higher number of perks. Surely better to be able to level slightly above the enemies, than be able to get to level 50 when the enemies stop at 25?
A challenge once in a while at endgame is fine.
But I seriously think that if Im not close to a demigod when Im level 50+ there is something wrong with the game.
There should be a payoff, you should get better and more powerful compared to the world as you level.
I think Morrowind made a good start to adress the god problem in their expansions.
I hear many people used to being a god installing bloodmoon or tribunal and being amazed at how fast they got their asses handed to them.
I would like it if at level 50+ there were a few bosses I would have trouble with, endgame bosses guarding fat loot or just there for the heck of it, but generally I should be one of the most powerful forces in Skyrim.