This is a multi-edge sword. If they made every level of armor with the same stats, I feel you'd get copycat builds with a quickness, and (I think) it will get boring. Many battles would come down to who misses first or who has what armor in which spot. Or, you simply have those that CAN get the legendary-ish boss armor, and they're magically instantly all better than everyone else (provided they level their gear properly, and level up themselves)
Also, there is the $$ aspect of the game. People assume that because it is a new boss armor "OMG, BOSS ARMOR! MUST HAVE!" without taking into consideration the ability to get the + version, or if it even has better stats. They'll spend money on gems for health refreshes, or energy refreshes. Then Joe player gets said armor and hopefully tries to max it out, thus ensuring Joe player keeps playing the game longer, and without realizing, there is just as good armor available through normal questing (aka "free"). People see a cool looking armor (which who doesn't have that "I think my knight looks pimp wearing this" armor that they keep around?) they want, and are willing to spend money for it, regardless of stats. If all stats were the same, and people were made aware on a large-scale basis, (you know, this forum perhaps, or simply smart enough) they'd probably stop spending money on them. People just see "ooh, legendary, **** armor! its the next best thing." Purely a money grab. So by keeping the stats fluid between levels, they can ensure money flow simply by releasing armors that look attractive as opposed to have the best stats.
Plus, the game by advertisement, is free to play. So if they only had the best armors as the purchasable/DPC chests (which many people would say it should be) it would be seen as unfair, and clearly geared towards the money spenders. People who have no intention of spending money, would probably quickly leave, thus really ensuring less money spent. This way, with some of the craftable armor with solid stats, it allows people to be competitive without having to spend money. The longer a person plays, the more likely they are to eventually spend money. A positive for the player base as a whole, as more people are around, thus more potential money spenders for Gree.
For many of us though, yes, many would like to see the best armors be the least attainable, only had by a select few. If that is the case though, then the gem farmers/buyers should have the advantage, and then the game becomes "(s)he who spends more, wins more" and that makes the game just suck for people who don't spend money. As long as there is a way to purchase more arena energy, more gems, and more boss energy, there will always be the people who will "pay to win" thus instantly making it unfair to those who don't pay. Does Bob with the $12000 credit line who blows $$ on the game in the form of "buying" his way to boss armor deserve to be better than Joe who doesn't spend a dime and tries to farm his way doing quests and fusing etc? Some think yes, some think no. In this case, maybe I'm far reaching in my thinking or theorizing, but by not always having the best armor be the boss armor, or that new super legendary, it alleviates some of this from happening.