Body and paint are the smoke and mirrors area of restoration. Almost everything else can be nailed down with a firm estimate. For their part, almost every car, or shell, a bodyshop gets needs more work than expected as the owner didn't completely strip and media blast the shell. That's the body shop's job. So many more horror stories are encountered. Plus there is no flat rate book to say how much job X can take and how much one should charge. That calls into play the capabilities of the person doing the work. John may be able to block sand a fender in an hour, whereas Frank may take 3 hours. So on both sides its hard to be accurate with estimates.
Most decent restoration body jobs and paint with average metal working are running near the $20K mark around here. There are quite a few who are saying no to a restoration and enjoying the cars.