@oratio
I think it is because if everyone did it, everyone would need a sufficiently sized farm, not just the fields, but home, storage, roads, etc., the field would also have to be large enough to accommodate fluctuations in harvest output to avoid starvation in a bad year and to enable storage of excess harvest, which means in the end, most of the time it would be impossible to “Just grow enough food to feed their family”. In most cases, there will be a lot of excess production. sum that up for a few dozen million families and the wasted food is tremendous.
Farmers that produce a lot to feed many people on the other hand will use the space they have much more efficiently, that means less food will be wasted and compared to the subsistence farmers, a lot of space that would be wasted for their excess harvest, storage and housing would also be saved, meaning less forests would have to be cut down, less pesticides would have to be used, less artificial fertilisers, etc.