It’s playable solo (and only solo in the current beta branch), but the devs have been nerfing that playstyle for years in the name of multiplayer balance. There are artificial limits on what one player can learn and do, with massive penalties to anything you didn’t start the game with. They include those nerfs in single player because they intend for NPCs to pick up the slack after they’re introduced.

(Note: NPCs have been a promised upcoming feature “after the next set of changes” for over fifteen years at this point.)

Fortunately they include settings to undo the learning speed nerfs, and hopefully will add more to make the upcoming crafting rework less of a pain for solo players. The litany of tweaks available at world creation are one of my favorite things about Project Zomboid, right after all the stellar business name puns.