To say browsers are more than a Javascript engine like V8 is something of an understatement. There are loads of APIs they provide like WebGL, audio and video decoding, the Web Audio API, device orientation/motion, inputs including gamepads, form controls, text rendering, geolocation, WebRTC, WebSockets, Web Workers, camera/microphone input, AJAX, storage APIs, even speech recognition and synthesis, and so on and on. Then they also do lots of clever optimisations like parallelism in rendering, loading, decoding, and so on. And then they have hundreds of engineers working on new features constantly, ranging from cryptography features to ambient light detection, battery status detection, MIDI support and more, as well as ensuring every platform works identically without any incompatibilities, and fixing all the bugs that inevitably crop up, often including platform-specific ones.
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!).