Game of thrones isn't the walking dead. The lore and backstory around the Night King and his army were fleshed out in dialogue as well as scenes throughout the show.

They were a subplot for GoT. I think too many people read too many fake leaks, fan theories and head canon to realize many things they thought are simply incorrect.

If Jon kills the NK it would be too obvious. Dany tries, after hearing from Bran they weren't sure if dragon fire works or not. That scene was really awesome too, it had a Terminator 2 vibe.

Jaime killing the NK would be cool for the king slayer arc, but considering he just joined the fight for the living it would of been pretty crappy in the end. I still firmly believe he will be the one to kill Cersei though.

There's always been things throughout the seasons people simply didn't expect or thought for sure they knew but got it wrong.