Or another way to ask: Why can’t Metahuman have muscles and real Human bodies like ML Deformer (with or without the deformation…) , Why are they not staying consistent?
It is more complicated to do that than it seems unfortunately. But this is definitely something that will be explored! 
Keep in mind that the ML Deformer sample was a character with a hand-made muscle setup for a specific character. To bring that into MetaHuman directly would require a lot more work from a technical point of view.