I assume because the feds calculated that they can collect more revenue by taxing elements of the tickets rather than the fuel, but I really don't know.

(Note that some states do tax airline jet fuel and some airports assess a flowage fee per gallon [which is for practical purposes equivalent to a sales tax].)

Airline jet fuel duty is covered by international agreements I think, presumably to facilitate trade.

I don't know if that also applies to purely domestic use though. Maybe that's how states can impose a tax?