Lee, parts sitting on the shelf is part of it, mostly it's the best profit center the dealer has. The dealer isn't selling much new stuff, bad economy and all. So parts and service have been the things that keep the dealer open. We don't have much choice when it's a dealer only part, you pay what they want, or you don't fix your tractor.
This isn't only heavy equipment, car and truck dealers have been doing the same thing for the last 25 years. When the Japanese started building assembly plants here in the mid 80's, they left the home plants open and building. So they created a glut of vehicles in the US. The dealers had to cut their profit margin to move all that iron, so the parts and service costs went up to keep the dealer in the black.
A Toyota rep told me 20 years ago about 'units in service'. This is what Toyota told the dealers; sell lots of cars and trucks, what you lose on new car profit you'll make up on parts and service.
Now everybody does it....