The reason is fun.
In a game like WoW where there’s not a lot of space or things to do that doesn’t involve “numbers go up”, this is one of those things that’s missing to break up the tedious cycle of run around and kill, go into instance and kill, do pet battles and kill, group with players and kill more players.
The usual for the game right now is tedious and with no breaks in between it gets a bit stale for a lot of people even for the people who basically live in m+ dungeons or pvp queues.