and you have sponsers that will in most cases donate or sell for cost the materials being used for the show and for the home's consumption. Then you have probably the show and the shows sponsers again paying some of the labor of the crew that is on the job. Then, I again would speculate that most if not all the homeowners are in the high income bracket that are doing this for themselves and the show. Then Bob's crew are probably not on that job all the time. They probably sub out the work and are only there for the filming and the walk throughs.
Your question is very valid and interesting and please understand that I am only speculating on this as I do not have any ideas what is going on up in corporate, but remember this is a business and a business survives purely from profits and sponsership.
Your homes are mostly on the New England Coasts where there is money and beautiful homes to work with, You will never see a home as modest as it is and as proud of it as I am, to ever be on the show. The house has to have features and old settings.