I read once that a lot of the animations that Sims use when talking to one another are tied, through code, to hand gestures that they make. To allow them to visit while both hands are holding the fishing rod, for instance, would involve either trying to "untie" the the voice animations from the gesture animations, which sounds quite involved, or to have the fishing pole appear and disappear rapidly so that they could make the gestures, which would be quite immersion breaking. I forget exactly where this was, but it stuck in my head.