Because people want the cosmetic stuff, which was proven time and time again in GW1. It's still rare and damn difficult to get - the dungeon stuff will require several runs through a dungeon on explorable mode, the cultural stuff is freaking pricey and generally you will not be able to afford it right away once you hit that level. And then there's legendaries that we haven't even seen yet but we know for at least one legendary item requires you to max out at least one crafting discipline.
It's not a gear grind - if you have crafted armor and I have Nightmare dungeon armor, mine isn't necessarily going to be better - because it's not required, but people will definitely be going for it, because the appearance is different and rare.
There's also the fact that the game changes continuously - different events pop up depending on how previous ones turn out, and they've said that they're going to tweak the content often, so if, say, you return to your starter area a month after the game launches, you may find a completely new DE there.
Stop thinking of the game as a typical MMO. It's not. If you just rush to level 80, you're going to find that you missed half the game. There isn't a typical end game to it because the game starts at level 1, not level 80, and they've said before that end game doesn't start at the end.
Also, the reason why most games have to have a gear grind, is that they have to keep people playing in order to get those subscription fees. There's plenty here to keep people interested without going "okay if you want to do that one area you need armor with +5 against demons which means you need to grind this area repeatedly hoping it'll drop for you".
