Calling it a math problem is pretty simplistic. Saying that power bonuses don't grow is also simplistic. There are some clear benefits at higher level such as paragon class abilities, epic destiny abilities and so forth. There are better powers at high level, toss on stoneskin and while your defences may not be as high the end result is you're taking less damage. You'll have more get out of jail powers to avoid crits, more feats to pump damage and you'll have more encounter and daily powers and magic items and can still be using them 5 rounds into the fight instead of being left with at-wills after round 2.