Do we know how many Expac's have come and gone, when this world boss showed up in the lifecycle of the game, and if they have methods to keep new content from power creeping old content? It's possible that world boss is something that really needed a dozen players back in the day, but now soloing it isn't a big deal if you've got a good comp.
Of course, it's just more likely that it's an anime MMO where protagonists get broken skills and abilities which means they can solo shit that would take 20 non Protag characters to get the job done.