Okay, Joseph, he’s been through some stuff. Like heavy-duty PTSD. His brothers beat and sold him into slavery, his boss’ wife tried to seduce him and cried rape, his boss sent him to prison, and spent two years longer in there than he should have thanks to the cupbearer he did a solid for. Sure, he came out on top, but experiences like that affect people for a long, long time.
When Joseph sees his brothers, it’s been many years. He now has a wife and children, it’s been at least 10 years since he last saw any of his family, and his last memory of them was not a good one. How do you begin to repair that rift? Is it even worth trying? As far as Joseph knows, his brothers despise him. He knows his father loves him, but how can he get back with his father?
If Joseph reveals himself to his brothers as soon as they arrive, they can choose not to tell their father. Joseph, then, will never see Jacob again. Even if Joseph’s brothers don’t actively despise him, as they did, they still won’t be inclined to fess up. “Hey, Dad, remember that time when we told you that Joseph was killed by a wild animal? Thing is, we kinda lied about that. We beat him, threw him in a pit, and then sold him into slavery and faked his death to you.”
Yeah, that’s not going to fly. Telling Jacob the truth would almost certainly kill him. Beyond that, what are the odds that the brothers are going to take responsibility for what they did? None. The only path to healing his trauma is to be reunited with his family, but the only way that’s going to happen is through some Machiavellian machinations.