Why I obtain an empty string?

(let [my-map {:a 1 :b 2 :c 3 :d 4}
      fm (java.io.StringWriter.)]
  (binding [*out* fm]
    (for [[k v] my-map]
      (println k "->" v)))
  (str fm))
;=> ""

(with-out-str (for [[k v] {:a 1 :b 2 :c 3 :d 4}] (println k "->" v)))
;=> ""