My take:
1. Until recently tractors were largely a "local product" similar to "mobile" homes in trailer parks. They got built, shipped, sold, moved to a lot then rarely moved far again.
True the tractor was driven all over the farm, but it was rarely taken far from the original farm, thence normally close to the original local dealer.
The local dealer would know the history of the tractor, thus could value it fairly.
2. Also tractors can suffer a wide range of use. Looking at the rust on my 2007 I was amazed how much rust cropped ip in some small places in two years. The guy obviously never put it under cover. My '88 Ford van has less rust and has never been garaged.
A "bluebook" would be difficult to accurately create and maintain