I doubt you'd have enough control or the right vantage point to strike a killing blow from the end of a rope. You need to puncture vitals or you'll just end up with a very angry mammoth with a spear sticking out of it. So "steering" the spear to the point of impact and reacting precisely in the last fraction of a second is important. A modern corollary might be trying to shoot clays by steering your shotgun with a rope.