Liberation does not happen by itself. Slave drivers do not let their slaves go voluntarily.
Pharaoh also hardens his heart and refuses time and again to let the Israelites go.
Liberation for Israel happens at the same time, and in the same event, as the judgment upon Pharaoh and his army. The judgment over the oppressor is the liberation of the oppressed. The liberation of the people (Exodus 15:2–13) occurred because God’s fury consumed Pharaoh and his army, and he shattered them (Exodus 15:6–7).
6 C'est pourquoi dis aux enfants d'Israël: Je suis l'Eternel, je vous affranchirai des travaux dont vous chargent les Egyptiens, je vous délivrerai de leur servitude, et je vous sauverai à bras étendu et par de grands jugements.