Why do you not make Champion Interactions "in advance"?
For instance, say you knew you wanted Xerath to say something unique to Taliyah. Or have Illaoi say something unique to Aurelion Sol. Why not record those interactions ahead of time?
2 Comments
Because themes are malleable, concepts change, names aren't nailed down until right at the end, and the champion pods work separately, so the VO isn't written in synchronicity with other champions. This question is like asking someone to write a critique of a movie that won't be released for several months. They'd likely end up writing lines that didn't make any sense.
The only real exceptions to this rule have been Rengar, because he was designed from the get go to be the rival of Kha'Zix specifically, Poppy's mentioning of Targonians and gems because Taric was in the process of being worked on and Targon stuff was already in the pipeline, and Aurelion Sol's taunt to Taric, because they were literally a patch apart and by the time Aurelion Sol's VO was being written they had already figured out Taric for Targon.
They actually did that a few times already (
has a joke that talks includes a gem-loving targonian,
has taunts for taric and 
has a special line for kha'zix even though kha was released a bit later).
However there is usually a huge gap between releases(and they rarely release related content close together) I mean Xerath got his last VO update during the shurima-event,Taliyah was likely not even planned at that point. And even if the champ is already planned there are a lot of things that can change during development(I mean aurelion went from "ionian stormdragon" to "targonian stardragon" and was delayed several times)