Because it goes against so many tested and verified scientific developments and the literal reading of the story presents objects and events that are unlikely or impossible.
"Everything had a begining. Nobody was around for that begining. Why is it itrrational to think that God created all things? What explanation is more logical?"
That the beginning can be determined through investigation and the compilation and examination of information, without resorting to the supernatural.
The beginning is going to have to be supernatural. We don't see the creation of the universe every day. It seems only the most illogically hardcore of atheists would maintain that matter is eternal. If the creation of matter is supernatural work of God, the creation of life is a small thing. Could not the creator of all matter create life in seven days, especially if He revealed that mankind? He certainly could have used the Darwinist method had He chosen, but the natural law we read about is that life begets its own kind. Jesus Christ endorsed the Genesis account as fact in Matthew 19:4. A lot of people who profess belief in Christ try to keep a foot in both camps by belief in a "theistic evolution". But this is the same as rejecting Jesus as an ignorant charlatan.