N2 nitrogen in the air is not useful to plants, it needs to first be “fixed” (converted to usable organic forms). Bacteria in the soil do this and so plants have evolved to just obtain their nitrogen from the soil. They could have probably evolved to do it themselves but there was no need as bacteria already did it.

This only becomes a problem when you want to pack more plants into a smaller section of soil and bacteria cant provide enough nitrogen. For the crop yields we get today we need fertilizers to make up for what the soil bacteria cant do.