Maybe he means why isn't it automatic so you don't have to code it in. There are several reasons. It's expensive in computer time so it's left to the author to decide how it's done. Some things might not need it, etc. Sometimes you don't want it, like a ghost floating through walls. Sometimes you want special things to happen rather than just, don't go through it. There are some engines that have automatic collision detection. The Blender Game engine, does. It takes some study, etc, but it's not too bad once you get used to it. Also, it increases the size of the library if it's automatic. When you are doing something like an applet, that you want to load fast, you want minimum code. JPCT does a large amount of things for the tiny file size.