My theory is because TS4 originally was meant to be an online game which in those you always have to create your "character" first before loading into a world/map. Like in Second Life (a virtual world where you control an avatar) you always have to create your avatar before you can access a "sim" (aka world) in the game.
If it was an online game there would have been no pre-mades for you to start playing with, because all of the sims you'd run into would be characters that are being controlled by other RL people. And without a character/avatar/sim you couldn't purchase/acquire land to build on.
