Most MMOs do this for two reasons. One, they start you off with simple tasks so that the more complex tasks don't confuse you as you begin to level. By starting you off with simple kill missions, they can teach you the basic mechanics of the game without worrying about overwhelming you with complexities. The notable exception to this rule is EVE Online, because it has a learning curve like a brick wall, which is accomplished by dropping you straight into the game with minimal instruction.
Two, they want the player to feel like they are participating in a story, rather than immediately being awesome. In any story, you have build-up, climax, and resolution (there's more to it than that, but for purposes of gaming, this paraphrasing works fine). The protagonist starts off as a weakling, and immature brat, or some other embodiment of a trait that needs to change. The antagonist stays the antagonist throughout the whole book, and is usually defeated at the end. Developers follow this exact same model to the letter.
You start off at level 1 and you do your boring level 1 quests. As you gain levels, you begin to experience more and more of the storyline, often developing into your own as a player, character, or in the case of roleplaying, both. This is considered the build-up. Then, once you reach a high enough level, you start fighting story characters and huge bosses and various other extreme menaces to the game world. This is considered the climax.
Here is where the literary train stops. You see, in order to effectively hook people, the developers have to provide the late-game with a perpetual feeling of accomplishment. They have to make the literary climax last forever. This is why you see a lot of addictions as well as a lot of burnouts from playing a level 70 raid 50 times in a row. This is also why you see a lack of early-game content. They are too busy trying to keep the late-game players on their perpetual high.
There's a change that needs to be done here. If MMOs are to improve, they need to follow the literary example even further. Much like a book, make the build-up interesting, not just the climax. Furthermore, add in a resolution for the players after they have exhausted the late-game content. A reincarnation system can go a long ways into keeping a player cycling through excellent content from beginning to end. By taking this focus away from late-game and changing it to a full-game focus, you can please players of every devotion and skill level. Because when you have players feel like they are actually creating stories, you'll build not only a great MMO, but devoted players for life.
Each week James Murff writes Under The Hood, a deeper look at MMO game mechanics and how they affect players, games, and the industry