Because many people are conditioned into believing that the only way God exists is by taking the Bible literally word for word. If Bible literalists were to ever find out that a certain story in the Bible did not happen they their faith is shaken to the core and then they begin to doubt many things. It's the premise of the stories that convey the truth....not the words of the stories themselves.
For example. The Flood is a creation story that was told by many many many cultures and at least a thousand years before the Hebrew people told their version of the story. Many parts of the story were different but some were the same. For example, the Assyrian story of the flood, a demi god hero convinces the angry gods to spare some people. Other accounts, such as the Welsh and the Transylvanian accounts give a more worldly down to earth account. A "flood story" is a way to describe an event in which Gods of the heavens poured down their wraith to destroy humanity. Water was a very violent and fearful thing in the ancient world. Moreso than fire. Also, one can easily surmise that an actual flooding did occur when the Atlantic ocean filled the basin of what is now the Mediterranean Sea.
The Bible is full of many stories that other civilizations had as well. For example, even the Christ story emulates that of the story of Mithra.
Mithra was a God Son born in the Persian Empire and the story was told seven hundred years (and well documented) before the Christ story.
For example, Mithra was born of a virgin woman named Anahita on December 25th
Mithra was wrapped in swaddling clothes, placed in a manger and was attended by shepherds
Mithra had twelve companions
He performed miracles
Mithra died and sacrificed himself in the name of Peace
Mithra was resurrected three days later
He ascended into heaven
In the ancient Assyrian and Persian texts dated 600 BC, Mithra was called "The Way, the Truth, and the Light", "Te Redeemer", "The Savior", "The Messiah"
The day that Mithra followers worship him in on Sunday (which was called the Lord's Day)
They had a Eucharist and a Lord's supper
Mithras stressed baptism
Sounds familiar

Does this mean I believe Jesus was a fake? By no means. There are two things I believe what had happened.
1. I believe that God gives us many people in this world to show us love through charity and that eternal life is granted through love.
2. Many many things about Jesus was added throughout the centuries. For example, Jesus did not come from a virgin mother. In the Old Testament (Isaiah) when the prophet proclaimed the Messiah's birth he mentioned the Messiah would come from a Young Hand Maiden. NOT a virgin. In the Ancient Hebrew, Isaiah says "Almah" which is hand maiden. If Isaiah wanted to say "virgin" he would had wrote "bethulah."
The Greeks at the time when they wrote the Old Testament in the Septuingent (LXX) they translated the word into "parthenos." In other words, they mistranslated it. However, world leaders of both ancient religions have concluded that it was not virgin but handmaiden. But, because of tradition for the last 1800 years, no one dared to changed it.
Sunday was the same way. Emperor Constantine used the stories of Mithras and the Roman's own pagan Sun Day holiday as the Lord's Day. Many cultures like that were merged.
Even with all of this said and done, it does not take away the validity of Christ's walk on earth, his message, and his power. Even the secular historian, Josephus (who wasn't a Christian) made historical accounts of Jesus' ministry, death, and rumors of a resurrection. So I do believe in the history of Jesus. I believed he was the Hebrew Messiah from the lineage of David. However, what makes me a Progressive Christian is the fact that I do not believe that Jesus was the only one who walked this earth to give answers in a world that greatly needed them.
Don't take my word on this. I encourage anyone to research and to study these things for themselves. Not just by going on the internet but by going to Universities and asking questions, ask to see documents and manuscripts. And, if you have the time, to learn these ancient languages yourself.
It is because I studied all these religions without people trying to hide things from me or try to cover them up, that makes me respect my spirituality all the while.
Christianity isn't perfect. We've had our darkest moments for 1800 years starting with Constantine. There's nothing to really brag about. Christianity has done a real number against people in the name of eternity and of God.
But I need to make some things clear with this. Never, in all my life, have I ever blamed God or Christ. When an entire German village had their women all executed for witchcraft, or the crusades, or when the Pope turned an ignorant eye away from Hitler and Mussolini, I did not abandon who God is. Christ, through God, gave us all the moral responsibility to love one another. If people truly loved one another then the Crusades would never had happened. The Salem Witch Trials would never had happened. The Spanish Inquisition would never had happened. Love is the most simplest commandment God has ever given to us but it is not to be treated lightly and it can be, at the same time, the most difficult commandment ever. To love your neighbor even when you think hating them would be justified.
As Ghandi pointed out, "I like your Christ. I do not like your Christians."
I read a commentary by Gandhi when he explained when he read the Sermon on the Mount. Gandhi wept and wondered why Christians did not do what Jesus preached. As a Hindu, Gandhi was more of a follower of Christ than the Archbishop of Canterbury was. Because Gandhi knew what it meant to love others.
Words cannot describe how alive the Bible becomes when you read it in it's original language. Read "Ruth" in Hebrew. It'll be like sitting down with a nice erotic novel.