Modern game engines also leverage SIMD and multi-core SMP. Javascript is inherently single threaded. While there is hope in retrofitting SIMD in (Rivertrail ParallelArrays or Dart's recent work with new SIMD TypedArrays), I don't see any path for supporting multithreading. JS is inherently async and singlethreaded in its memory model.

WebWorkers are not really a multithread model. Game engines could be written to leverage message passing based isolates, but in some cases, but doing it in an optimal form isn't exactly developer friendly, it's one of pains of developing the PS3 SPEs is that they didn't share system memory and you had to DMA stuff around.