Because they have learnt about religion from a young age and believe it while they learn about evolution from a much older age. Also because some places like parts of america teach creationism in their schools.
And on the issue of respecting other people's views...you can respect their right to have that view, doesn't mean their view is right. Evidence for evolution - loads, evidence for creationism - 0. I have no problems with people believing in religion, everyone is entitled to their own opinion but as someone who is planning on studying evolution for the rest of my life, when someone gets into an argument with me saying I am wrong and evolution doesn't exist and creationism is right...well sorry, it's not a question, evolution does exist.
Rant over. (I like this thread, I can rant and know i'm not going to be bombarded with 100 creationists :P )