Rav Tzvi Pesach Frank strengthens this question by noting that the Shu”t Rashba (1:18) explains that no berachah is made prior to performing the mitzvah of giving tzedokah because of a concern that the poor person may refuse to accept it. In the case of returning a lost object, this logic would not apply since the item already belongs to the individual to whom it is being returned, and he will surely accept it. Nevertheless, Rav Frank points out that it is possible that the owner of the object had already been me’ya’eish from ever recovering it, in which case there is no obligation to return it to him, and due to this concern, we do not make a berachah.
Alternatively, he cites Rabbeinu Bechaye, who maintains that blessings were not established for the performance of logical mitzvos, a category which includes the returning of lost objects.
