If you're not looking for a genuine answer from a Christian, skip this.

First thing: the translation of "the fruit of the knowledge of good and evil" isn't really that good of a translation. It's closer to "the right to define good and evil. That means that eating the fruit is basically saying "fuck you, God. Imma do my own thing". That's not how God designed humans to live, and is incompatible to living alongside someone as powerful as God, which is why God told them not to eat it.

But why create that tree in the first place? Essentially, choice. When you're in the supermarket and you see 50 different flavors, but everything is from the same brand, do you really have any choice? Same thing with God. Unless you have the option of rejecting God, choosing to him means nothing.