The Rashba (Shu"t Siman 18) and the Shu"t that are attributed to the Ramban (Siman 189) explain the reason why we do not make a brocha on mitzvos such as Matnas Kehuna and Machatzis Hashekel: Whatever he is giving is not really his, rather Hashem gives it to the Cohen, as it says in Divrei Hayamim (1 29:14), "For all is from You, and from Your hand we have given it to You". (This is in contrast to mitzvos like Trumas, Maasros, Hafrashas Challa and Pidyon Haben were he can make a brocha on the separation, whereas here he would be making a brocha on the giving).