Why don't I just program my own engine and save the 5 percent?

Take it from me, if you want to create games within a reasonable time frame and you don't have a large team of dedicated engine programmers to help you, you'll want to focus your efforts on what you sell (your game).

Not having to focus on programming a game engine gives you the freedom to think only about how to make the actual game. Not having to maintain and bug-fix your own engine is a load off your mind too.