Why we not adjust ovility of Pipe from outside surface?

Firstly I want inform that I am not a Professor, However I can answer your question as per my experience, Pipes are used various purpose if it is used in very high pressure transporting liquids it should be corrected from the inner surface, because the pipe thickness may be vary in the oval shaped area therefore it not recommend  correct from the outer surface. Other vise  it may give some pressure losses in this area.