Because a part of the experience that they want for the game, one of the MMO aspects, is the feeling of getting that loot that you've been trying for. If people could just stick with the same weapons for the whole planned yen year run of Destiny, it would give them less rain to experience the content. It would rob them of feeling that sense of accomplishment and even joy when they finally get that gun that they've been striving for. Am I disappointed that the Fatebringer that I finally got in June will be largely obsolete? Sure. But I understand that there will be new weapons coming that could be as good or, dare I even hope, better than it. I think we should head into TTK with wide eyes, and see what has been built for us to experience. Then, once we have our hands on those weapons, is the time to debate their worth against year one weapons, not before we've had time to try them. Just my $0.02