Why not?
Because a sinful human being cannot save other sinful human beings.
Because believing in a Jesus that is only a great moral teacher is just a false religious belief. That Jesus is like the unkown god, in Athens, in the Apostle Paul's day. This is what Paul said to the Athenians about their false belief (Acts 17):
22 Then Paul stood up at the Areopagus and said: 6 "You Athenians, I see that in every respect you are very religious. 23 For as I walked around looking carefully at your shrines, I even discovered an altar inscribed, 'To an Unknown God.' 7 What therefore you unknowingly worship, I proclaim to you. 24 The God who made the world and all that is in it, the Lord of heaven and earth, does not dwell in sanctuaries made by human hands,
25 nor is he served by human hands because he needs anything. Rather it is he who gives to everyone life and breath and everything. 26 He made from one 8 the whole human race to dwell on the entire surface of the earth, and he fixed the ordered seasons and the boundaries of their regions, 27 so that people might seek God, even perhaps grope for him and find him, though indeed he is not far from any one of us. 28 For 'In him we live and move and have our being,' 9 as even some of your poets have said, 'For we too are his offspring.' 29 Since therefore we are the offspring of God, we ought not to think that the divinity is like an image fashioned from gold, silver, or stone by human art and imagination.
30 God has overlooked the times of ignorance, but now he demands that all people everywhere repent 31 because he has established a day on which he will 'judge the world with justice' through a man he has appointed, and he has provided confirmation for all by raising him from the dead." 32 When they heard about resurrection of the dead, some began to scoff, but others said, "We should like to hear you on this some other time." 33 And so Paul left them. 34 But some did join him, and became believers. Among them were Dionysius, a member of the Court of the Areopagus, a woman named Damaris, and others with them.