I think you're forgetting that the function can have more than one recursive call site within its body. A for-loop is identical to a recursive function containing only one call site within its body.

By contrast, in order to calculate the number of nodes in a binary tree, you would do

    (nodes (tree)
        (if (tree? tree)
            (1+ (nodes (left-node tree))
                (nodes (right-node tree)))
             0))

which is not like a simple for loop at all.