Beds as opposed to seats mean fewer passengers per train carriage, so the fixed cost of operating the train is shared among fewer tickets.

You can make one trip per day with a night-train, whereas a seated train runs 16-20hrs per day. So again, fixed cost of the train existing is shared between far fewer ticket sales.

Then you need to store the train during the day, and you need to securely park the train when it arrives at its destination at night (a normal train yard isn't secure enough if you have a hotel full of people, and a train station usually needs all of its platforms).

Hundreds of kilometres of good quality electric railway infra is expensive to maintain. And by good quality I mean capable of relatively decent speeds like say 120-160 km/h. With airports you need to maintain lot less.

I think last time I looked it was that jet fuel is exempt from taxes in the EU? but trains aren't so flights are artificially cheap

Jet fuel is barely taxed and most touristic places subsidies the tickets to bring in the consumer cash cows.

Pretty much yeah.