Tomb Raider loads a pre-rendered scene that is the same every single time you start the benchmark thus you can compare it across systems. That's the point of it and hence became a staple alongside with other games (some games are even notorious about it because that's their only use case [0])
WoW doesn't have anything like that. And everything can affect the frame rate from the mobs in the zone, the weather, the unique players with unique transmog, the raids are different every single time etc.
It's not just loading the game and there you have X framerate and that's the score, that's not a benchmark.
0, https://www.kotaku.com.au/2016/01/the-game-thats-more-known-...