I mean...he doesn't? First time he mentions it (after doing a very small genocide in Egypt to get them to stop slavery (Interestingly non-military slavery is very rare in ancient Egypt, only really existed in the middle kingdom) he's like "Nope, you gotta let them go after 6 years unless they want to say".

Now admittedly he then says his wife and children remain slaves, but in fairness god is trying to gently introduce to the people of Israel the concept of being human. There's a theological argument in some parts of Judaism that god chose them because they were the literal Worst, and if he can beat them into shape the rest will be easy.