Creationism is just a myth that was developed in order to comfort people. Thousands of years ago people didn't understand where they came from or how. People don't like not knowing the purpose of their existance. Leaders at the time couldn't explain anything, so they simply said "God did it" to put people's minds at ease. This is not unique to Christianity. All cultures and relgions have stories that explain the creation of the world. They all vary greatly, but their purpose is the same. They served as an "explaination" in the absence of truth.
Ancient Babylonian mythology says that the world was created when a god named Marduk kills the monster Tiamat. Tiamat's wings are torn in half and become the earth and the sky. Later Marduk kills Tiamat's husband, Kingu, and uses his blood to create mankind.
In ancient Japan it was believed that the islands of Japan were created by two gods, Izanagi and Izanami.
The Inca believed that man kind was created when Con Tiqui Viracocha rose from Lake Collasuyu and then created the Sun, Moon, and Stars. Con Tiqui then made more humans from rocks, including woman who were already pregnant. And then he sent humans to all corners of the world.
Hindus belive the world exists with in the dreams of Vishnu who wakes up every 4 billion years or so, bringing an end to the universe when Shiva appears.
Creek (not to be confused with Greek) mythology suggests that humans were created from the clay on a hill called Nunne Chaha.
Ancient Greek mythology is a long complicated story involving many gods playing out a plot that more or less resembles a modern day soap-opera.
The Māori creation myth tells how heaven and earth were once joined as Ranginui, the Sky Father, and Papatuanuku, the Earth Mother, lay together in a tight embrace. They had many children who lived in the darkness between them. The children wished to live in the light and so separated their unwilling parents. Ranginui and Papatuanuku continue to grieve for each other to this day. Rangi's tears fall as rain towards Papatuanuku to show how much he loves her. When mist rises from the forests, these are Papa's sighs as the warmth of her body yearns for him and continues to nurture mankind.
Norse mythology says that Odin and his brothers used Ymir's body to create the world. His flesh was used to make the Earth and his blood to make the sea.
These cultures and their stories were around LONG before Christianity ever even existed. And certainly any rational mind would agree that they're all just fantasies. They have a great deal of cultural value, but none of them are true. So, what exactly makes the Christian mythology, which is more or less an amalgamation of older beliefs, any different?
I'm sure you won't accept my answer. In fact I'm mystified as to why anyone who is already convinced his beliefs are right would bother ask the question you've asked. But whatever. I've presented my arguement and if it doesn't reach you it most certainly will reach someone else.