Having grown up driving tractors with hundreds of horsepower, it's not that. These projects are neat, but they are backyard gardening toys. If you wanted to build a tractor that competed, you could start perfectly fine with something big. You would need the kind of machine tools that many farmers have to repair their own equipment and the capital to build something big, but most of what you would need that a person of decent skill couldn't do themselves is available off the shelf.
Someone could disrupt with a small scale competitor, but the place to start wouldn't look like a go-kart, it would look like this [1] 50-150 HP range, probably without a cab, with appropriate standard hitches, hydraulics, PTO, etc. You could learn lessons and scale from there. The couple of open source farming tractor things I've seen have been cute, but just not anything like what would actually compete, and there's no reason not to start with something that at least is in range.
You don't need massive machines, you need a certain ratio of crops to labour and capital.
If the go kart tractor allows experimentation with automation and presently difficult to automate farming methods or crops then it may be viable.
It won't compete with the ten hectares of wheat needed for one person to earn a living wage even without other costs, but there might be a niche for produce rather than calorie crops that looks like 1000 gardens rather than a vast ocean of corn.
Edit: Opened your link to the rather small tractors, this is what I had in mind with the one in the article being more along the lines of what the reprap was in the 2000s compared to a ratrig now.
Someone could disrupt with a small scale competitor, but the place to start wouldn't look like a go-kart, it would look like this [1] 50-150 HP range, probably without a cab, with appropriate standard hitches, hydraulics, PTO, etc. You could learn lessons and scale from there. The couple of open source farming tractor things I've seen have been cute, but just not anything like what would actually compete, and there's no reason not to start with something that at least is in range.
1. https://www.caseih.com/northamerica/en-us/products/tractors/...