Because they are dead? Ghosts exist in Horizon verse you know....they're largely comprised of Ether

This is kyoukai senjou no horizon dont question
but one thing i can think is that if they have the power to go to heaven cant they just become ghost when they die.

Horizon is more of Fantasy rather than Sci-fi...

Why does Nate calls Toori ''My king''? is it because of that promise (in episode 9 if i am not mistaken there is a flashback when toori says that when he will become king of musashi he will make everyone's dreams come true) is it because of that? and i want to read light novel but on baka-tsuki there are volumes that are half-translated or untranslated at all and do you guys know in what volume does season II anime ends because i want too keep up with the story in light novel

I don't know the exact answer to your question regarding Nate, but I think that is the case. IIRC someone did say her promise is addressed sometime later. Might be in the 3rd story arc which is where the story continues after the anime.

Season two adapted both 2A + 2B. So you'd be starting with what is translated so far of vol. 3A. Then patiently wait for further translations.

You could always read 2A for any scenes that got shorten or cut in the anime adaption of S2. It has been a fun read overall imo.

Arigato :3