Two things:

* There is no way to evaluate your own performance in combat as there are no official DPS meters or any other means of seeing your output.

* Until the 8-man raids nothing in the game actually pushes players to do more than button mash.

The problem ultimately stems from a combination of these two points. A very large number of players in this game are bad because they have no idea how bad they actually are and the game itself never pushes them to improve.

This problem would go away if we had more challenging content in regular dungeons and such. As long as the game is made as easy as possible to cater to these players they will never get better.