How? Give me an example of how the government is preventing someone new from competing in the tractor business.
The laws aren't preventing someone from starting a tractor manufacturing business, though we could veer off and talk about regulatory capture and speculate that any new business that John Deere can't prevent legally will simply be purchased by them before it becomes too big.
But I challenge you to start a tractor repair business that specializes in late-model John Deer tractors. You may technically start one and hang out your shingle, but you won't be able to execute any meaningful repairs without running afoul of the DMCA. As you might read from the article, John Deere has taken advantage of the law to squelch competition in the very lucrative repair business for their equipment.
But hey, if there's nothing stopping you as you say, you should give it a try. Quit your day job, start your John Deer tractor repair business, and get back to me in six months and tell me how you're doing. If you're correct you'll be filthy rich with farmers throughout the nation clamoring for your services.