The Sims is a game where the player decides personal things for a Sim (CAS, aspiration, career, skills, friendship, relationships, marriage, affair(s), pregnancy).
You don't want anyone to interfere with your decisions, would you?
Let me give you an example:
You create a Sim (Sim A) and visit your friend's town. They meet a Sim (Sim B ) there and fall in love with them. Then you exit the game. When you enter the game again Sim B is already married and has to kids with Sim C because another player decided to play that way.
Here is another example:
You want to play rags to riches style and start out poor. You have a minimalist home and your clothing style is sloppy. When you enter the game again your Sim is suddenly living in a mansion. Okay they reached their goal but you - as the player - didn't do anything.
