Hell is eternal because of the debt we take on when we sin. If someone owes a debt and you throw them in prison for it, they can never get out because there is no way for them to pay it back while in prison. That’s what hell is.
Thankfully Jesus paid our debt and gave us a positive balance. Also why it’s so important for us to forgive others, because of how much we have been forgiven. God is God of the living and not God of the dead, which is why we must make the choice to repent while we are alive. Jesus provides a living way to redemption and to the promised land.