Writer's conceipt - a good story needs suffering. The oligarchs might need some human assistance with, for example, software engineering to operate parts of their vast holdings. Note that in the original Manna there were still some human workers - lawyers, at least. Even if it was just 1% of the population, that's a significant number. Note also that the POV character was generally unaware of the wider state of the dystopia, and so were we, the reader.

The workers supply not only the drama of suffering but also a (meagre, absurd) customer base for the fast food restaurants themselves.

Last but not least, given the long distances involved in interstellar travel, an oligarch must delegate their authority, either to a machine, a human, or a combination, and that is an opportunity for some drama as experience and vision inevitably diverge. This would be true even if, for example, the delegate is a perfect clone of the oligarch. It would be within these cracks and crevices hope could form, only to be crushed, in artistic, brutal fashion.