If we made our own engine, to maintain compatibility it would essentially amount to writing a new browser. For an idea of how hard that is to do even with a larger team than us over a long period of time, look at CocoonJS - it has lots of problems ranging from bugs to compatibility issues and missing features like the Web Audio API and WebRTC. It does the job for iOS, but I doubt we'd do any better ourselves. Meanwhile Crosswalk (albeit with a few initial launch issues that are being worked on) is doing a much better job on Android, because it's a real browser engine. And hopefully iOS will soon get their act together and implement some decent browser technology for apps, but on the other hand Safari itself is already pretty good (just needs WebGL!).