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.
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.