While you don't have to pay slaves, they were not free. They were expensive to buy and you had to provide food etc. (And they might rebel and kill you.) The Rom

While you don't have to pay slaves, they were not free. They were expensive to buy and you had to provide food etc. (And they might rebel and kill you.) The Romans did use things like water mills, so they might as well have used steam engines if they had actually been practical. I'm not sure the Romans ever realized the device could be used to perform work. It is really just a curiosity.