Bruce asked this in response to my last post, and it's an excellent question. I think this is typically the way we respond to these things. Why did God set it up that way? Why make it so that it's so easy for Adam and Eve to fail?
Here's where my interpretation of Genesis drives people a little nuts. Because I don't think this is a story about what happened back then. I think it's a story about what is happening now.
So the fact that God places the tree of knowledge in the garden is descriptive of the present, not the past. The question to ask is, why did God place the tree of knowledge (so to speak) in YOUR garden? Why is that option always there, where you can choose to rely on your own knowledge, your own choices, your own tendency to rely on yourself? Because the garden of Eden is really about you and me and where we find ourself. That tree is in the story because it's in your story. The tree is in the middle of the garden because it's in the middle of your life.
So as we watch God's desires -- and human rebellion against God's desires -- we have to ask, What is God's desire for me? Why am I always fascinated by the other options besides obedience to God? Why do I choose to rely on myself?