Why a solo campaign using a published module works

Partly, its because 5e relies way less on the DM than 1e does, and because 5e is way less open ended than 1e.

The published adventure modules, Storm King's Thunder in my case, basically do half the work of the DM.

It feels like I'm reading a novel, only the novel's not shit because I play the role of the protagonists and my chance of success or failure, and the consequences and setting thereof, are decided by the dice, the adventure book, and the Dungeon Master's Guide.

It takes reading 3 books - the PHB, the DMG and SKT - simultaneously to properly play solo, but at my novice level of playing it's still a lot of fun.