In every MMO, there's a ceiling. The ceiling is the hardest bit of content with the best gear available at the moment. Every MMO has one, and every MMO has its ceiling raised as new, more challenging content is introduced. This is MMO 101. Everyone should know this. Very few people will partake in content if there is no reward. So let's say for funsies, there is a raid instance harder than the current rank 50 one. But this raid instance drops no gear, no items. There is zero reward. Its only purpose is to give you some use for your new gear. Well, once players realize there is no reward for completing this super hard raid, they won't care to partake in this content.

One thing that always accounts for the best gear, is PvP. But we don't currently have PvP, so that isn't a valid use for the biggest, baddest pieces of equipment you can get.