I think it has to do with the fact that a lot of players will pick the highest difficulty options for the sake of purely achievements and such, and then go on forums and complain about how slow and challenging it is, forcing the developers to take action.
What would work is just giving people an option to get level 100 chars for free infinitely. Then any complaints about leveling being slow and hard wouldn't be legitimate, because technically leveling will be instant and challengeless for those who want it. But then, it is unlikely to happen, since level 100 char purchases is one of the sources of income for developers.
Ultimately, I just think that that is the modern MMO developers' mindset: "Give people an easy time so they can get all the rewards without putting in much effort, so they will be hooked on the endless flow of rewards and never stop playing". The fact that things like immersion, fun, challenge disappear then is secondary, since they are nothing next to profit. Which is perfectly reasonable, mind you, every developer is concerned first and foremost with profit, that's the only way to stay in business for long. Sadly, it also makes the product itself less than it could be otherwise...
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On a side note, IMO, scaling would be a terrible idea. The problem with scaling is that you don't feel becoming much stronger, which is pretty much a core RPG element. I would hate to be in a situation like one has to be in games like Oblivion: "I just leveled up, and the enemies scaled with me, but my gear didn't change, so I became weaker".
Maybe a more gradual power growth would be more reasonable. So a character of level 60 wouldn't completely obliterate mobs of level 50, but just would have a significantly easier time fighting them.