Plus the 'the engine doesn't do what is needed' of course. Which is kinda a big deal, and why ED, NMS and X4 all use their own engine. And which is why SC has been, and will remain, in such a mess. But when you ignore the 'actually releasing a game' part then yeah, there is no point really to using your own engine.
Btw, there are also excellent reasons not to put fuel into a plane: it saves money, fuel is bad for the environment, it increases the weight of the plane, can provide a safety hazard and so on. As a small sidenote, it is kinda needed to get the thing in the air. Its also needed to keep it in the air. But yeah, lots of reasons against fuel, too.