While it's true that better gear technically can come from dungeons, it comes only when you happen to get lucky on what drops. While leveling, you're very unlikely to get a full set of green dungeon gear to drop for you before out-leveling it.
Maybe you'll get one piece of a green dungeon set on-level if you're lucky (or in extremely rare cases, maybe two), but you'll still need another source for the rest of your gear. Crafting (if you can HQ the items) lets you have the good gear as soon as you reach its level. That will be the best gear available until a green item drops for you, which might not happen for a long time. In most cases, it won't happen until after you've leveled up through another tier or two at least of crafted gear.
