There are several problems transforming the curvilinear perspective of the wide angle GoPro lens to a rectilinear one.
The transformation stretches the image in certain areas and compresses it in others. This lowers the image quality as new pixels are created as approximations and interpolations from the information in the original image.
There is also a problem with the rectilinear image from a very wide lens. To make sure that lines are straight certain sacrifices must be made. The closer an object is to the edge of the frame, the more stretched it will appear. This may create unwanted forced perspective effects.
These are a couple of reasons to why the GoPro won't automatically correct for the distortions that the wide angle lens creates. Also if you want to correct for them in post, there is nothing that stops you from it.