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Hey there. Here are a few problems from what I understand:
Topsoil is teeming with microbes, bacteria, and beneficial fungus. Topsoil takes a long time to form (decades, centuries, or even millennia). When it gets tilled deeply by tractors it disturbs that topsoil by exposing it to the sun. If the soil isn’t covered by something (cloth) a lot of the top soil will just blow away which doesn’t happen as much when it’s untilled. In nature plant foliage falls to the ground and eventually it breaks down and puts nutrients back into the soil. What a lot of farms do after harvest is remove all the plants that are done fruiting and either dispose of them or make compost. If farms do that long enough earthworms, microbes, and fungus no longer have the food they need to survive (unless the farm is ‘building the soil’). Another possible problem is not rotating crops properly. If you grow certain plants in the same space year after year, tomatoes for instance, you’ll find that the soil becomes depleted of the nutrients that tomatoes use to grow. Ideally farmers after harvesting a crop will plant a cover-crop of plants like alfalfa or clover and it gets tilled back into the soil before the next planting. Farmers can be greedy and not put in the effort to utilize cover-cropping and the soil pays for it.
This is way over-simplified of an answer but shows some basics of how I understand soil being overworked.