I'm not familiar with the details, and don't have the references to hand, but WASM has been designed in a way that reflects the design choices of Chrome's Javascript engine rather than optimizing standalone compilation.
Part of optimizing for the design choices of browser engines is to make it easy to emit machine code. The browsers have multiple compilers in them, and one of the compilers has the task of emitting simple machine code quickly, so the code can run immediately (reducing start-up time). In V8, this initial compiler is called Ignition.
