Because a lot of games are failed and base everything on LEVELS.
In UO you had the whole game world available to you as soon as you created your character, there were no amusement park rides (or dungeons) that said “You must be this tall to enter”. No, you could freely enter them and die horribly, or use your own craftiness and skill to avoid the monsters, and instead get horribly killed by 3 Dread Lord Playerkillers, before they looted your body, carved up your corpse and then ressed you, only to kill you again, while you shouted profanities they could only see as “oooo ooo!!”. But you had the CHOICE, you could go anywhere, and experience danger and adventure, and kill people, or get killed, by people or monsters, or even a Bear you decided to attack to get your wrestling skill up to 60 and hopefully gain some STR so you could wear plate armor.
In the LEVEL Based MMOs you start at level 1. The end level might be 50, 60, 75, or whatever. As you gain levels, you will notice that you need to move from Tiny Rats, to Bigger rats, that give basically the same XP in %, but as you outlevel the Tiny rats, they will give less and less XP, due to the level difference, but they will also be easier to kill. You will notice that the old dungeon you visited, and perhaps liked to do, no longer offers any challenges to you, but the new dungeon will offer you a challenge, or just be piece of cake, for another level or two, until it too, is outleveled or outgeared. The LEVEL based MMO Is filled with “You must be this tall to enter”. And to enter everything, you need to be at endlevel.
At the end-game, you will not be able to level any more, but the content will no longer scale away from you, and sooner or latter, everyone you played with or enjoyed grouping with, will be on your level again, and you can enjoy doing the dungeons or raids with the same group of people again, without being restricted by a level difference. At the same time, if you decide to do PVP, at end-level, you will not be oneshot by someone, simply because they are 2 levels above you, and due to how the stat works in the game, you might have a “inbuilt” 8% miss chance vs someone 2 levels above you, whereas they will be unable to miss in addition to having another “tier” of gear or new talents. In a LEVEL based MMO - player SKILL is disregarded and replaced with TIME/LEVEL/GEAR.
Simply put:
In a game that has LEVELS on the characters, the only equal playing field is at max level. And that is the only place your friends wont outlevel you, or you they, so you can do the same content together without taking massive XP penalties for grouping with someone much higher (or lower) level than you.
EQ2 tried to remedy this situation somewhat by allowing people to group, and allowing the highest level players to “Mentor” the lowest level player, where he’d get more XP, and they’d get “Scaled” down to his level (but keeping some of their massive gear bonus, to keep themselves awesome).