It takes a huge amount of energy to break the bonds between the atoms of nitrogen in the atmosphere. The bond is so strong that concentrated nitrate fertilizer is a high explosive. Fertilizer bombs are very real, and fertilizer factories sometimes explode The city of Lebanon was devastated when a warehouse full of fertilizer exploded. Basically, in order to make nitrogen based fertilizer, energy is pumped into nitrogen, and it is capable of releasing that energy with catastrophic speed. Some plants- mostly in the legume family (beans) can partner with bacteria in their roots, they pay the bacteria energy (sugar) in exchange for nitrate. But it is so expensive, most plants evolved to just use what is present in the environment.