There is a contradiction, its as clear as bloody day....
You are talking about games that are designed for consoles, all the examples you have listed are based on the consoles limitations. From memory (and will probably need some corrections as I am not a expert) the difference between a sandbox game and a mmorpg or any mmo tbh is going to be whats going on behind the scenes. Those sandbox games, you used gta4 as an example so i will to. You said it has to display a crapton of character and car models however these all operate on an algorithm written into the games code so nothing happens without a trigger of some kind be it reaching a street corner or entering a park. where as in a mmo outside of the limited NPCs everything is unscripted characters move randomly may log out or have a large log in of players all around you. all of these factor into the memory usage of a game, then there are the models to be rendered. so we have to wait for information from the server to reach us (this is related to your internet connection, but still somewhat relevant i feel) in that time have the players moved, have more people logged in or out/entered the area you are currently in, changed gears etcetc. so your rig be it pc or console is tracking and processing all of this info. From this I assume that a simple sandbox such as gta4 is incomparable to a mmo.

There is more that could be added and if someone here cares to add and elaborate more or even correct me that would be much appreciated however its 430am here and I really can't be bothered to type more.