I will say they do have scaling on many powers through feats and through attribute bonuses and other things which combine to increase effectiveness. When I was calculating how much damage a fireball did in spite of lower numbers of dice many static modifiers were involved and brought the damage far closer with elemental bonuses and so on.
I am not sure why they didnt decide to have more scaling powers. I certainly would have preferred it. Did they think as separate powers they could simply have more... and scale flavor if they wanted? nuclear blast ... vs Fireball.
Yeah, so, just thinking in crude terms about it, suppose you are a striker, like a rogue. You start out with a Daily that notionally is about 3[W] and then you get a bonus d6 on top, plus you get probably a static +5, and possibly a couple more points, with your W starting at D8, lets say (IE its a rapier, pretty solid starting point).
At epic the same 3[W] power is leveraging a D10 W, 3d8 SA bonus, +10 for DEX, +6 enhancement, and another 10 points or so from feats. So it is doing MORE than 2x the basic raw damage, but then you probably have larded on some fairly nasty additional stuff, like maybe Frostcheese or some sort of Radiant craziness, and AP bennies, and then some way to get bonus attacks out of it all, or enhanced crits, or something. Not all that will reflect in that single power though. Plus you undoubtedly have a higher level power with 5[W] and some crazier effects/conditions added on. You might still use the lower level power, maybe, but probably more because it ties into some combo that you've developed.
In terms of the 'why'... I think where 4e actually 'went wrong' (if we can even say that) is in the whole "give you new powers at every level" formula. It leads to a design with a HUGE number of discrete powers, far more than can ever be made distinctive in any meaningful way. So, then once you wed yourself to that formula, its redundant to scale existing powers, aside from at-wills a bit and here and there, mostly class-feature related stuff like Turn Undead that you're not going to replace.
And this really is my main 'beef' with 4e, it just has too stinkin many powers! This is the reasoning that took me down the path of design in my own game. Why are there so many powers? To fill each level with some! Why are there so many levels? Good question! How do I reduce the need for all this filler, AND make each level meaningful?
So, I cut the game back to 20 levels, 10 less levels of filler material required, and the tiers are simply each a bit smaller, big deal. Nobody much ever made it through 30 level-ups anyway, so why not cut it to 20? It just works! Next, provide scaling for at least many of the powers instead of brand new powers. My goal was to have less powers than 1e has spells (around 300 in core 1e) total. If you assume you need about 10 classes to cover all your bases, then it works out to about 1.5 new powers per class per level. Lets assume there are 2-3 fairly differentiated builds per class, and lets assume you offer 2 'power ups' for each power, so any given character only needs to have 10 discrete powers during their 20 levels, that means you can basically make one full discrete 'build path' worth of powers for each of 30 builds, or 3 builds per class, or you could even build 15 classes with 2 builds each.
Now, obviously some players will chose some mixture of elements from your basic builds, or come up with their own variations, mixing multiple classes, whatever. That should provide a good solid 50 or 60 viable mechanical build paths that a player oculd follow, and that doesn't account for reflavoring and whatnot. Given that sort of fairly restricted initial list, then you can pretty easily construct new variants too, it only requires roughly 10 to 15 new unique powers to make the workable core of a new class, plus enough 'feats' to get you 2 upgrades to each of those powers (and many of these feats will apply broadly to multiple powers and classes, so you probably only need a few actual new feats to support a whole new class).
I think you can get close to the build diversity of 4e, with its 1000 core PHB1 powers and 200 feats. Definitely if you were going to reproduce all of the PHB1-3 and a couple other 'core' classes you'd need 1000 powers even this way, but that's drastically short of the more than 10 THOUSAND powers 4e has now (the current compendium lists 9409, but I seem to recall the count being somewhat higher, they may be missing some).