I too really dislike the world boss system, it is completly unclear when or how to spawn one of these bosses, so far i know they are part of a long fate chain, but their spawn cooldown is absurdly long, and also the fates are not shown on the map, so you dont even know when its there.
I think i have killed the world bosses just enough times during the expansion to get all rewards, that means one kill in 6 months.
They might spawn when i am at work, or sleeping, or just not logged in and then it is a week long cooldown to happen again.