Ah, but that's where you're missing the implications of world-class magic, the MMO system, and the one that they have now. The adventurers would produce that disgusting purple blob if they used the menu to make food, the same way Sword Art Online did. It's true that is does turn purple if their cooking level was too low, but the real world equivalent happens all the same, it's just not as immediately obvious.
That said, boars don't automatically turn into a large bone with meat on it, and goblins when killed don't turn into bubbles and coins. Think of the MMO-aspects and menus as a world-class spell continually running. As far as we know, People of the Land have no access to that kind of information. They can't talk telepathically (chat/whispering), and they definitely can't "click" on someone and figure out their name, rank, level, and class. When adventurers aren't cooking or crafting properly, and try to use the menus to do it, they're using the world-class magic as a shortcut, and of course it doesn't execute properly.
Similarly, I wouldn't expect People of the Land to kill a few monsters, and automatically have access to a new skill. They would have to learn and study magic the hard way, or practice with a sword.
However, I think adventurers are in the same boat. They can't learn any new skills outside the shortcut system and it's UI without doing things the hard way, or worse, that the system will actually hinder them if they're not the correct class/subclass. The same isn't true of People of the Land. Lenessia could become a spellsword with little difficulty if she practiced both, with her subclass as Princess.
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If Shiroe wonders if he can create very powerful documents, I seriously want to know what things Serara can do once she maxes out housekeeper. It has to be secretly better than it seems.
edit: Perhaps you literally cannot clean up a guild hall, hindered by the system itself. Maybe Serara will become quite valuable over the months, and charge exorbitant fees! Not that money matters to adventurers...