You must differentiate between engine development and game development. Two pair of shoes.
But event the most difficult things, such as AI, Compiler/Interpreter or Physics-Kernel is not that difficult. The point is that you do not have to invent anything, it is all there, because all game (and engine) development are outlined by academia at least five years in advance.
Writing software is NOT difficult. Figuring out the mathematical, physical and / or numeric is the real difficult thing. But games do not have to do that...