The expectation for magical loot or wealth by levels came about due to the attempt to provide a scale for measurement of monster power vs PC power so that the DM will know what monsters can challenge PCs of a certain level. In older editions, there was little to no such system. DMs guessed or just picked monsters due to his preference and PCs either run away a lot or died often. Any comparisons that were done was mainly by eyeballing monster HD, but this system was very vague and incomplete. In 3.x, the more detailed CR system was developed and tried to take into account monster abilities, attacks and defenses etc. The encounter and monster building system in 4e is basically an extension of that system.
Now, when the monster vs PC scaling system was developed, the game designers need to make certain assumptions and the availability of magical gear was one of them. You either take into account that PCs of level X had Y amounts of gear, or you don't.
If you balanced it such that that monster of level Z is an appropriate challenge to PCs of level X with gear Y, then PCs of level X will require Y gear to face monster Z. This leads to expectations that as PCs go up in level, they will accumlate enough gear so that by level X, they will have Y amount of gear. If the level X PCs have less then Y gear, then the power scale is thrown off and they will be less powerful then what the system expects when facing a monster Z.
If you leave gear out of your assumptions when comparing monster vs PC power and balanced mundane PCs of level X to monster Z, then any better gear that the DM give the PC have will make them more powerful than what the system expects when they're facing monster Z.
In both 3.x and 4e, the designers went with the PCs will have magical gear assumption and developed the balancing system based on that assumption. This of course led to the wealth by level expectation in 3.x and the treasure parcel system in 4e. If the DM wanted to go with a low gear game, he'll have to recalibrate the monster vs PC power scale to fit the new assumption. In 3.x, this was a mess to do because there was a lot of power bound within the gear and the math was not transparent. In 4e, gear has much less power and the math behind the gear assumptions is a lot more transparent so it's much more obvious on how the DM should modify the rules in order to run a low gear game.