The gist of why libertarians and ancaps desire less government power is that they prefer more voluntary and decentralized means of governance than by one large central government, that is why some ancaps prefer using the term “voluntarism” to describe what they think. Corruption would also be a lot more difficult to deal with in a government as you are putting all of your eggs in one basket and relying on just the one, while in a society with no government that has multiple companies corruption simply means one competitor is out of the game