GMs don’t tend to know much about how engine coding and netcode actually work. 99/100, your guess is as good as theirs.
But most of it stems from relevance. The more players/mobs that are near each other, the more data has to be calculated and sent to each player. Starts going up astronomically once you get above a certain number of them. Vale invasions have a ton of mobs and players and since it’s so small, people and mobs stay relevant to the player for longer, which means the server has to relay more info around.
This is why games like Fortnite weren’t possible until recently (to be stable and have a high tick rate).