Because if everyone has the same gear it might as well just become cosmetic. Gear serves two purposes at the higher lvl. Both a reward for clearing the content/reaching a milestone and as a way of pushing even farther. If you remove the reward then all that is left is pushing content and while some do it just for that it destroys the feeling of progression. Your character is not getting stronger, the bosses are not getting harder its just stagnant and that will ruin the game.