It would probably be a lot more easy to just bake animations for walking, punching etc. rather than calculating all the torques necessary to do these types of things. The forces involved in just standing up are pretty complex, and you don't want to waste CPU time having your character just stand.
You can use physics simulation when the person is actually at the mercy of physics, like when he is dead, or thrown back or something. That is called rag doll physics, but most games don't bother simulating physics for ordinary tasks.