And more to the point, if you want to use synthetic fuels, why on earth would you pick hydrogen?
Yes, it burns to clean water, but if the carbon feedstock is renewable, synthetic hydrocarbons are renewable too. The efficiency loss from doing the additional steps to build hydrocarbons is not large compared to the efficiency losses of using hydrogen, and storage can be so much easier with something denser.
I'd assume because it is complicated. Capturing enough carbon, splitting it, generating enough H2, combining it with the carbon to make long enough chains. That all sounds complicated and expensive and probably needs even more surplus green power that we don't have. It also doesn't solve the problem of local pollution when burning carbon based fuels.
Methane has good energy density, doesn't demand cryogenics or diffuse through steel, burns very cleanly, and can be used in modified gasoline ICEs - without even sacrificing the gasoline fuel capability.
Isn't the point that it is as simple and convenient as normal gasoline and also that you can use your gasoline car? If you are using gases it is a hassle for everyone and you need a new car or a full retrofit. At some point we have to ask ourself why we would even do that. Is it really worth it compared to just using a battery?
Without cryogenics, methane has such low energy density that a low-pressure fuel tank would still have to be as big as a bus for your compact methane-powered vehicle to go as far as you could on a few gallons of gasoline.
Yes, it burns to clean water, but if the carbon feedstock is renewable, synthetic hydrocarbons are renewable too. The efficiency loss from doing the additional steps to build hydrocarbons is not large compared to the efficiency losses of using hydrogen, and storage can be so much easier with something denser.