Sam2 -Two reasons: (Maybe only one.)
From Halachafortoday:
“1) There are a few conditions that must be met before a scent will require a Bracha before smelling it, details of which we will now begin discussing.
A Bracha is only recited over a scent that has a tangible source (referred to in halacha as Yesh Bo Ikar). In other words, the item that is being smelled must be the item that is emitting the scent. (Shulchan Aruch Siman 217:3)
Therefore, an article of clothing, a food, or a utensil that was near something that has a scent and absorbed the scent does not require a Bracha when one smells the scent on the clothing, food or utensil. If one recites a Bracha on such an item it is a Bracha L’Vatala (See Mishna Berura 217:15)
2) Man-made scents that are not natural, and are manufactured synthetically, is a subject of debate amongst the Poskim if a Bracha is required, thus, due to the rule of Safek Brachos L’Hakel (whenever it is a question if a Bracha is required, we rule to not say a Bracha, in order not to say Hashem’s name in vain Chas V’Shalom) no Bracha would be recited on such a scent.
Thus, most of the perfumes, colognes, body sprays etc. that are on the market today would not require a Bracha as they are produced synthetically to acquire a certain smell, and aren’t naturally fragrant. (See Shmiras Shabbos K’Hilchasa Perek 61:12 and footnote 232 quoting Rav Shlomo Zalmen Auerbach Zatzal. It is important to note that even natural perfumes that would require a Bracha would only require it when smelling it in the bottle, and not when smelling it after it is already applied to a person[on whom it is permissible to smell], as it would then be considered “Rayach Sh’Ain Bo Ikar”. See also Shu”t Ohr L’Tzion Perek 46:54 and Shu”t Avnei Yashfeh Vol. 2 Siman 16)
Potpourri (which is basically flower petals, bark and other materials which have no intrinsic smells, and are sprayed with perfumes and other synthetic smells) does not require a Bracha when being smelled as it isn’t naturally fragrant.”