Because it would be really expensive to start a new tractor manufacturing business; it would take years to even get prototypes up and running; you'd still have to prove that your tractors were as good as or better than these 40 year old antiques (which they probably wouldn't be at first); you'd have to be able to make something that could meet safety and efficiency standards of today while giving up the advantages of tight control over maintenance; you'd have to figure out how to compete with the dealer, mechanic, and parts networks that represent a huge advantage of the existing players; and you'd have to figure out how to do all of that without running out of money. It's probably a twenty-year project. You up for it?