>Would you be willing to elaborate why "energy per unit mass" matters when the mass in question is a completely different substance with different cost and availability profile?

Sure. Specific energy (or energy per unit mass) between different types of materials makes a huge difference. For example (Source here[0]):

   Material   Type of generation   Specific energy (MJ/Kg)
   Hydrogen   Fusion               639,780,320
   
   Coal       Oxidation            24.0-35.0

Note the specific energy of a Kg of burned coal compared with a Kg of fused hydrogen. Fused hydrogen generates roughly 200,000 times the energy per unit mass than burning coal.

You're right. Cost and availability play into this as well. There are estimated to be ~1.06 trillion tons of coal on earth[1], hydrogen is the most abundant element in the universe and even makes up a significant amount of the mass of coal.

Burning hydrogen/hydrocarbons is, compared to fusing hydrogen, an incredibly inefficient process.

I'd say that being able to generate 200,000 times the energy per unit mass is an important consideration.

As for availability, hydrogen is more abundant and cheaper to produce (unless you have petatons of plant matter, the right conditions and a few tens of millions of years at no cost to you) than any fossil fuels. Or just about anything else.

[0] https://en.wikipedia.org/wiki/Energy_density#List_of_materia...

[1] https://www.worldcoal.org/coal-facts/what-is-coal-where-is-i...

Edit: Clarified availability.

It's not that cost plays into it, it's that cost (broadly defined) matters, and the metric you have chosen doesn't matter. As a consumer I don't care how energy dense a fuel is; I care only about what it costs me to get that energy.

You might try to argue high fuel energy density implies low cost, but this is clearly not the case in general.

Exactly, and fuel is only a part of the consideration.

Though it's also true that the infrastructure to process, deliver and store large amounts of fossil fuel comes at a higher cost than is usually considered (as some of it is subsidised and socialised), plus there's the pollution.