Because this is a "software as a service" product that actually operates sort of like a service. Technology moves fast, and the company that makes the game engine is continually adding those latest benefits for you, especially Unity and Unreal. And again, the risk of building your game is now back-loaded. Unreal is totally free up front, and Unity is dirt cheap up front (sometimes free). The revenue share model means you only pay royalties once your game passes a certain threshold of success, so their incentives are aligned with yours. This is unlike most consumer products where you may never make any revenue off of what you create with them at all, and you're paying up front. The closest analogy would be if your car was free but they only took a percentage of your proceeds if you're a professional driver, and only after you made about $10k in a quarter.