It doesn't NEED natural gas as input, there are other ways, but natural gas as input is saving a massive amount of energy, possibly near 10x current energy input, by not having to split hydrogen from water. And fertilizer production is already one of the most energy intensive processes we have, so increasing that multiple times over is a huge amount of energy.

Basically hydrogen production is the limiting factor in producing clean ammonia from the air instead of using fossil fuels, and clean (and cheap) hydrogen production is far from a solved problem.