Because weight matters a lot in an airplane, and hydrocarbons are the densest form of fuel. Adding 20% to the weight of a car isn't really a big deal; adding that to a plane makes the entire enterprise fail.

If they can find a way to transform carbon-neutral electricity into a hydrocarbon, then they can keep airplanes going without having to burn fossil fuels. But it's hard to make that efficient enough to be economically viable.

> Adding 20% to the weight of a car isn't really a big deal

And typically the car can't go as far on a single charge as a tank of gas. I can usually go about 200 miles on a typical charge in my (cough) "300" mile EVs, but my last gas car could go about 400 miles between visits to the gas station. (But I don't care because I just plug in when I get home.)

That being said, I once rented an Infiniti that could barely do 200 miles on a single tank of gas.

To get back to the point: Batteries are just too heavy for airplanes, so unless there is a major breakthrough, synthetic gasoline is currently the most promising way to make airplanes carbon neutral.

An aircraft burns fuel and gets lighter as it flies its route. Batteries are always heavy.

The difference in energy density is much more important.

The specific energy of gasoline is 46.4 MJ/kg, while that of a Samsung inr18650 Li-Ion cell is 958.1 kJ/kg. Even accounting for the much lower efficiency of a turbofan engine the difference is quite significant.