Why doesn't Jaw surgery widen the ramus? Is this possible via osteotomy? Widening only occurs in some cases.

That is a good observation on your part. There are growth patterns which have adequate ramal/jaw angles development associated with a recessive mandible. But more commonly the lower jaw is deficient on both projection and width. However when we advance the mandible some of us do augment the width at the angles at the same time with implant materials for the best cosmetic result  One does not do it with altering the osteotomy because that is less effective  and could injure the TMJ.