Imu was waiting for the mother flame to flood the world again.
I’m sure plenty of shounen had the final villain introduced early, Demon Slayer is the first thing that comes up to my mind. Muzan could have wiped them all out if he just tried hard to locate the hashiras‘ base. He thinks they’re not a threat to him since he slaughtered them for hundreds of years. This is a similar situation with Imu and the World Government. They probably got complacent through the years.
Imu twerks over joyboy all the time
There are prophecies about his downfall
He could've removed the bases of support for a reincarnated joyboy with ease anytime he wanted
He has teleportation and dozens of top tiers at his service with the ability to make more with the flick of a finger.
It literally takes him like half a day to have all the god knights and gorosei teleport into any yonko base they want and demolish them.
Like maybe BB is partying and boom 7 top tiers with dozens of high tiers appear on his island.
Could have destroyed the allied joyboy nations like Wano and Elbaf with ease too.
Not to mention he didn't focus on the Nika until he awakened.