I can see why you might think that - but people and machines and groups are free to leave if they want - at least one of the Culture stories is based on this: e.g. "A Gift from the Culture".

Free-to-roam pets are still pets. The Minds are benevolent dictators, and generally seek not to be seen as such by humans.

What would the Minds do if all humans wanted to leave?

Probably sublime... who knows!