Multiple reasons. 1) Many people play the Sims with role play in mind, and make up their own stories for it. If the Sims spoke English, that would interfere or potentially contradict the storytellers. 2) Cost and space. Not only would more dialogue need to be recorded, but additional space would be required, especially on European discs that generally have 5 languages on them. Writers would also need to be hired. 3) Most importantly, no matter how varied the dialogue, it would quickly get old and boring. Turning speech into a background noise lets you focus on the goals in the game instead. It also places more of an emphasis on emoting and physical humor, making the game visually easier to track, especially during fast forward sequences.