I don't believe it is even the engine must be WebKit; it is that third parties can't implement a JIT-optimized Javascript engine like JavaScriptCore. So the br

I don't believe it is even the engine must be WebKit; it is that third parties can't implement a JIT-optimized Javascript engine like JavaScriptCore. So the browsers are stuck with either abysmal performance numbers due to interpreted javascript, supporting their engine having different javascript behavior on iOS, or shipping a skin over webkit. The skin approach is by far the least amount of engineering.