One reason is the issue of safety. Your muscles aren't warmed up on your first set, or possibly even your second in a training session. If I am training chest, my first two sets are pyramids as far as the weight used, and my goal is to prepare the muscle for the increased weight I will soon be using, as well as instigate deeper muscle fibers to trigger over the course of a workout. Obviously, a beginner who really isn't going that heavy in the first place will find this as less of an issue than someone who plans on ending their last set with over 400lbs.
After those initial sets, I still only train "close to failure", which means I end the set when I feel I only have enough strength to rerack or lower the weight under control. Without a spotter, it would be unwise for someone to get under over 400lbs on any exercise and train until absolute muscle failure unless they were in a power rack.
What all of this means is, if I train my chest and use three different exercises, I am still avoiding hitting near failure on the first set of any of those. This is because of the weight I use and the fact that muscle tears or joint injury are often the result of not preparing a muscle group for a movement with a slightly lighter weight before hitting the absolute heaviest weight you can move until muscle failure.