I think this was more popular/common 'back in the day' when barrels were harder to come by, and almost nobody - not even gunsmiths - had reliable access to borescopes. A 'shade tree' smith could rechamber an old barrel for someone a lot faster/cheaper than getting a new one in. The 'new' chamber/throat probably shot better than it did before being deemed 'shot out', so there was a lot of 'good as new!' B.S. propagated. Usually by the time the barrel stops shooting, there's been enough damage to the first *several* inches of the rifling that getting set up and indicated for a good, concentric chamber job would be challenging, at best.