‘It Comes to Include a Fragrance’
A person must recite a beracha before eating (beracha rishona) and another beracha after eating (beracha acharona). A mishnah (51b) states that sometimes only a beracha rishona is required. The Gemara explains that the mishnah’s statement refers to smelling a pleasant fragrance. A beracha (i.e., “borei minei besamim”) must be recited beforehand, but no beracha needs to be recited afterwards. Rashi (s.v. “reichani”) explains that a beracha acharona is not required because smelling is only a han’ah mu’etes (a limited pleasure).
Quick and Immediate
The Kol Bo (cited by Sha’arei Tziyun, Orach Chayim 216:3) offers a different explanation. He writes that a beracha acharona must be recited within a certain time frame. For food, the time frame lasts from the end of the meal until the food is digested (when the person still feels satiated from his meal). The pleasure of smell, however, doesn’t linger. Thus, the time frame to recite a beracha acharona after smelling an item expires immediately.
No Bodily Benefit
The Meiri (Yalkut HaMeiri citing Berachos 42b) offers yet another reason. Smell benefits the soul, not the body, he writes. Therefore, one cannot say a beracha acharona that includes the words “l’hachayos bahem nefesh kol chai – to give life with them to the souls of all that live.” Fragrance does not offer any life to the body.