Man the answers are all over the place on this one. Maybe because what your asking sounds more like a philosophy question . Plants can absorb nitrogen from the air btw. I'll assume you are asking why they can't turn it into protein. The short answer is they absolutely could but lack the genes for it. Why can't you turn sunlight into sugar? Why can't we breathe under water? The answers to questions like this is that we don't know for sure. Evolution took the paths it took and here we are. Maybe it's not worth the bother to plants when they get enough nitrogen from bacteria or lightning strikes. Another part of your question involved farming. Farmed plants are not natural. We use fertilizer to increase the yield, but those plants will still grow without fertilizer.