Because the evolution of the animal ended up with that muscle not being needed. Evolution is not perfect, with a lot of dead-ends and half-assed results. It is a process. I don't think you've grasped it. Natural selection only causes a change to become fixed in a population if the change is beneficial (provides more offspring, in essence). Natural selection "tries out" individual mutations, but if the specific mutation for the new orientation doesn't work out, natural selection acts against it immediately. It doesn't engage in wholesale replacement throughout the entire population and _then_ decide it wasn't a good idea.
So if the current plantaris lacks a function in this orientation, according to evolutionary theory there still had to be a function for it in this orientation in the past. Otherwise it would never have migrated in the first place.