Firstly, as you may already aware by the fact that you havent had to submit a personal profile and gone into sd’s office for multiple interviews and character analysis…this game hasnt been made for you personally. shock
So bearing that in mind, the xp system allows all players (it will help new players alot) to slowly build up their skills, letting them play a bit of the game then when they get used to it showing them more…so as to not overwhelm them.
But i guess thats wasnt really what you were asking, youd like to have 10 characters that level up together, even though they are different people, that way you only have to get to level 20 once.
This is probably (other than it just making sense, when i play rugby my friend who isnt playing doesnt get better at it too…) to satisfy the players who like having goals to work towards, those who like to play lots, the players that try to get rank 15 prestige on cod or unlock all achievements on halo.
But back to your question. The answer is its better for the vast majority, its a utilitarianistic decision, which has to be made in games. But you have the choice. No one says you have to create 10 characters…i think 3 or 4 is enough. And it wont really matter what level they are once they are above 7 which will be extreamly easy to get to. so really 3 x 7 = 21 or 4 x 7 = 28 so you only need to level 8 levels more than before, except these levels go much much faster.
Granted if you want the random fancy perk that makes you whistle when you skip then you have to put the work in to get it. But you get all that you will need off the bat.
You cant expect to have everything unlocked to you immediately, it would overwhelm you, put off new players, offer no insentive for those players that like to play for something other than fun.