While it's true, a cheap car has better paint than a good tractor, tractors are not exposed to the kind of road salt a car has to be built for.
The base paint needs to have a molecular bond with the clear coat, to have good chip resistance, and durability. To form a molecular bond, all the paint needs to be applied within a short time frame. That can't happen if you clear over the cured factory finish. So, yes, you can have your tractor clear coated. No it's not really a good idea. There is a fair chance there will be adhesion issues, because proper prep, would be difficult to impossible in some areas.
A high end base/clear system can have a very high level of durability. But, I doubt any manufacturer is going spend the money to use high end automotive paint, or a fleet finish, on a tractor. The increased cost would certainly put them at a disadvantage in the market.
If they wanted to spend more on paint, they should start with increasing the corrosion protection first. (That would be the undercoats, i.e. primers). Most of the time, they seem to be using direct to metal finishes, that offer limited protection against corrosion. You can live with a certain degree of fading, a lot more than corrosion.