Because they profit from the whole power structure. The pirates kill each other and Imu keeps ruling over the world with no issues.
Sure, but the Yonko are a huge threat to the WG's supremacy.
- Control large parts of the new world
- Including Wano, which is said to be incredibly important for the history and stores Pluton
- Have the Road Poneglyps
- Gorosei ask whether it's wise to anger Kaido
- Cannot stop angry Red Hair pirates, get pale as Shanks and Whitebeard meet
- Allowed Roger to conquer the Grand Line, reach Laugh Tale and start the age of piracy
- Im is visibly stressed and angered by Luffy and Blackbeard, tears their posters apart being butthurt, yet does nothing
It doesn't seem like his power is
that encompassing. I mean, it was literally said that the Marines (Garp) Teamed up with Roger to beat Xebec and "protect the Celestial Dragons". Why not just blowing up both Roger and Xebec along with their crews? Yonko would've never happened in the first place then.
I don't buy the "the WG just wants to sustain the order", when the WG is perfectly willing to destroy islands and kill thousands of people before having just a little bit of information leaked.