The reason they don't have brains is because trees aren't going anywhere. So if you're a tree, you're stuck where you are. Now, if that's a good place to be, then fine. On the other hand, if human beings don't like where they are they can move. In order to do, however, they have to be able to figure out whether there is better than here .
Human beings have a sensory system to do that.
As far as the tree is concerned, not having a sensory is actually a good thing. The Association for Supervision and Curriculum Development.