[ZeroGravitas]

Because it piggybacks on the political might of fossil fuel producers who are subsidized by taxpayers and don't have to pay for their externalities.

It's not "cheaper" for a mob connected waste disposal firm to dump waste into a river. If they were prosecuted for the damage they did to other people's property and had to pay for the damage they did then they'd need to raise their prices to the point that just dealing with it properly would be cheaper.

In physics terms:

Dig up hydrogen connected to carbon. Separate the hydrogen from the carbon. Release the fossil carbon into the atmosphere as CO2. Capture the same amount of CO2 from the atmosphere. Seperate it from oxygen, store the carbon. Use hydrogen to make Ammonia.

Versus

Use electricity to split water into hydrogen and oxygen. Use hydrogen to make Ammonia.

[ClumsyPilot]

So whats the best case possible efficiency?

[Sinidir]

https://www.sciencedirect.com/science/article/pii/S187661021...

This is a nice paper on the different fuel cycles of hydrogen ammonia and methyl.

Roundtrip efficiencies for hydrogen and ammonia are both at about 30% plus. Don't know how high it could theoretically go.