As far as why doesn't a new tractor manufacturer start up, they have to compete with a few things: first, the market for old tractors and parts (if they are repairable basically forever and the amount of land used by agriculture isn't changing, this is not a growth market). Worse, it's much more difficult to design a small engine for sale that is both reliable and will pass current environmental regulations. This is a major barrier to entry that didn't exist decades ago.
So if you're a startup in the ag space, it makes eminently more sense to make small parts for someone else's platform, which is why you see companies doing things like deploying GPS, cameras, lidar, datalogging systems. There, the market opportunity is "all tractors could benefit from this", the competition is initially "other small companies", the exit strategy is "get purchased by a large manufacturer who would rather buy than develop the tech themselves", and the regulatory environment is much more friendly.