Why does a farmer sell the wheat to the miller instead of milling it and selling the flour?

Why does the miller sell the flour to the baker instead of baking bread and selling the bread?

Because any additional step requires resources and knowledge that one can hardly have, together with capitals to make the necessary investment: running a mill is profitable if a large pool of users can be served, so that the needed large quantities can be processed, and so on.

Same goes for your adventurers.

Why does a farmer sell the wheat to the miller instead of milling it and selling the flour?

Why does the miller sell the flour to the baker instead of baking bread and selling the bread?

Because any additional step requires time, resources, and knowledge that one can hardly have, together with capital to make the necessary investment: running a mill is profitable if a large pool of users can be served, so that the needed large quantities can be processed, and so on.

Same goes for your adventurers.