The answer is simple. Most food crops have increased in yield steadily over the past several decades. The increase in yield is obtained by increasing plant density and increasing fertilizer. This results in higher yields, but on less soil. So the plants are relying mainly on the fertilizer for nutrition. The result is that the food grown is less nutritious.
As an analogy, suppose that a human being is fed protein powder, vegetable oil, sugar, and a vitamin supplement. That diet has the macronutrients and micronutrients that a human being needs. But it would not be at all healthy. You can’t expect plant crops to provide excellent nutrition when the plants are grown on ever less soil, and are mainly fed from fertilizer.
We need to grow less food on more land, not more food on less land. This will make plant crops more nutritious. We also need to make sure that the farm soil itself is healthy, with plenty of organic matter throughout the soil.
Growing ever more food on ever less land also makes crop failure more likely. To get very high yields, a farmer needs to make sure that every factor affecting yield is just right. And this makes obtaining very high yields a precarious proposition. It becomes ever more likely that yields will fall sharply, if everything does not go just right. That is not what you want.
Growing less food on more land provides food security. If more food is needed, we can increase the plant density and fertilizer, and produce more food at will. But when we grow more food on less land, there is no margin for error. An agricultural and food disaster becomes ever more likely.