Before responding to that question, let me turn it around. Why, given that there are no laws preventing them from doing so, are so few people in the 'modern world' engaged in by-hand cultivation, or even cultivation using draft animals?
One big reason is that land isn't free, and the cheapest land is located far from city markets, as well as from the amenities cities provide. And, even if there were still land for homesteading, there's the matter of potable water, housing, tools, seed, food enough to last until the first crops come in, canning supplies and equipment if part of that crop has to last through the winter, and some means of transporting a portion of the crop to market. If you're growing one of the local commodity crops, you can almost leave off the transportation part, since there will already be a network in place for that purpose, and you'll only need to move your crop as far as the nearest node in that network, but you'll also be competing with other producers benefitting from economies of scale for the thin margins they're accustomed to. If, on the other hand, you're growing watermelons in wheat country, you'll have to provide your own transportation.
Okay, back to the original question, why robots, given that there are people in need of work?
People are slow. One person can just about manage five acres by hand, if they work at it full time. Depending on the climate and what's being grown, that might be enough to feed as many as twenty, or at least provide them with vegetables, which isn't such a bad consumer/producer ratio, but it's far lower than the average for conventional agriculture. One person using the sort of equipment you see everywhere in rural America, can work between 600 and 1,000 acres, planted to crops like wheat or soybeans that lend themselves to bulk handling.
People are expensive. Who's going to pay the unemployed to produce by hand a small fraction of what is already being produced far more cheaply by means of machines under direct human control? Even if the current cost of production were to rise several fold, current practice would still result in cheaper food than what workers earning minimum wage could produce by hand. Granted that the economics are better for crops that don't lend themselves to linear mechanization (performing field operations while/by moving over the surface), and that a diet limited to those that do would be significantly impoverished.
People would be needed. A conversion from tractors to small, autonomous machines almost certainly would not result in a net displacement of workers, rather the reverse, and you're more likely to find enough people (4..8, at a guess) willing to maintain (and augment*) the machines that manage a square mile of land than you are to find enough people (about 100) willing to work that land by hand.
(* Especially at first, it's unlikely that robots would be able to do everything that needed to be done, and the gaps in their capabilities would have to be supplied otherwise, most likely by people.)