Could you explain why this is?

Water vapor in the stratosphere has a very high radiative forcing. Offset somewhat by particulates in the upper atmosphere.

Cirrus clouds and contrails have a distinct, and large, additional forcing.

What do you mean by this? What else than burning the fuel contributes to the greenhouse effect?

Building the airplanes? Servicing them? Heating the airports?

That's nebulous, are we going to claim that for all industrial processes?