So you grow the crops, harvest the crops, the government takes the crops and in return for you working on their project they will give you enough of the food you raised to survive.
That is not exactly voluntary.
You give the crops to the Pharaoh because that is the way it was done. Hindsight is a poor excuse for an argument. It was considered voluntary by the people, whether you agree or not. It was their culture which lasted for 3,500 years - more than we can say to day about our ‘culture’.