Because the world content rewards are not high enough to qualify for any other content (either due to Blizzard’s tuning or community issues).
Until SL, any type of player could feasibly gear up by enjoying the content that provided the rewards. But starting in SL, content doesn’t always provide appropriate rewards, or in the case of Torghast any rewards at all.
Blizzard has intentionally created these walls where the base-level rewards are locked behind pre-made group gatekeepers. WoW was never, ever like this before. Assuming that devs are not incompetent enough to intentionally kill their player base out of arrogance, why else would they do this other than to stimulate the token market?
Because players need to become more powerful over time, even if they exclusively do world content.
Just like Mythic raiders “need” that gear to drop after they have downed the last Mythic boss of the tier, or key runners “need” that Great Vault every week to fill out their set even after achieving KSM.
This is just how RPGs work. SL devs are so desperate to drain every last drop out of their player base that they forgot to consider the possibility that millions of players would want nothing to do with either the infinite anima cosmetics farm or the newly prescribed three endgame pillars.
Note: I doubt that even the so-called solo players are truly doing world quests exclusively. Some of them would even want to do group content if they could qualify for it. Any other content out there, including legacy group content, is made a whole lot easier with better gear.