Price is a primary reason. Another is to keep the original barrel profile. Perhaps it has express sights and a barrel band you’d like to keep. Keeping the original profile means no stock modifications to accommodate a new barrel if you didn’t have the profile duplicated.
In my limited experience, two rebores and having been around a half dozen fellow shooters who had rebores, I’ve not seen one that shot poorer after reboring. My 35 Whelen, originally a 1941 Winchester 70 30-06, shot well. Nothing spectacular but well enough. After Cliff Labounty rebored it to the Whelen, it thinks it’s a varmint rifle.
I had a Sedgley Springfield 30-06 with a really poor bore. It shot accurate enough but was really rough. JES rebored it to 375 Whelen and it shot about the same. No loss of accuracy but no noticeable gain.
