I have a suspicion that originally the plot wasn't going to involve Ultra Beasts. Like maybe the original idea was the Lusamine was obsessed with the box legendary, and she was experimenting on Cosmog to get it to evolve or call its parents, and maybe Type:Null was for batting it. This gives even more reason why the climax is at the Alter.
Then they decided to put their Ultra Beasts idea into the game, but didn't want to change the plot too much. So Lusamine is now obsessed with Nihilego, and she just wants Cosmog because it's an Ultra Beast that can open portals to Ultra Space, but she otherwise doesn't care about it. The hero and Lillie are the ones to go out and fully evolve Cosmog so they can get to Lusamine.
This is why only Nihilego gets any real appearance during the main story, while the rest of the UBs get shoved into a post-game quest. Probably someone came up with the idea of the UBs coming out and rampaging, but they didn't want to deviate from their outline of you fighting Lusamine at Aether Paradise to save Nebby, then chasing after her and finishing things at the Alter on Poni Island, so the other UBs only show up in a short cutscene to make things seem more serious, only to then disappear until post-game.
Solgaleo, Lunala, and Necrozma look more normal compared to other UBs because they were originally just going to be normal Pokemon, but were made into UBs so their plots weren't completely disconnected.
