Moses brought Joseph's bones out of Egypt when God freed them. Joseph made his family swear under oath that when God pays them a visit, they have to carry his bones with them. Joseph mentioned that surely, God will take them to the land He promised Abraham, Isaac & Jacob.
Maybe, Joseph's brothers tell this to their sons & daughters so even after x number of years, they are still reminded of Joseph's request. The Israelites know that the time will come when God will take them out of Egypt but maybe they just didn't believe so He needed Moses to perform the miracles, that they might know Him, trust Him and believe in Him.
Sometimes, we know what God said and what God promised us... but when it's already in front of us, it's hard to determine that God is already moving. Why? Because we are comfortable with the misery that we are in to. It dulls our senses and makes us forget the promise of God.
Be careful with where you are right now. It's not permanent. Don't lose hope. Keep the faith. God is moving.