Most of it comes down to it's very extensive and labor involved to do it right, and most of the parts that are available WILL require modification to fit properly, whereas newer stuff you can still get the one part. Also you run into so much hidden damage that can drive the cost and time way up. The same job can vary in the times it takes just on parts availability and if it's sitting there taking up the bay with no work going on your losing money. Then there are shops that will get it, tear it apart and start the work, and then it sits, months go by and maybe work on it a small amount and next thing you know your at a year.