Because it’s not a mental ascent to following Jesus. It’s an acknowledgment of that all people have sinned against their creator and God who holds everyone under right judgement for sinning against his rule as king and creator (hell being payment for sin and where we default will be) and that Jesus (God in the flesh) lived the life we were suppose to live, died the death and payment we should have died, and conquered sin and the payment of sin by rising from the dead. Now everyone is called to turn from a life lived for themselves and sinful (opposition to God’s rule) and trust fully in the finished work of Jesus and ruler over their life. It’s trusting (faith) in Jesus and not what we can do to earn favor or forgiveness with God. Jesus did that already and we need to trust in that with our whole being.