Why is no blessing recited by the Mitzvah of Kibbud Av Vaeim:[1]

A blessing is not to be recited upon doing an act of honor or fear of one’s parent, even though that upon doing so one fulfills a positive command.[2]

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[1] Rashba 1:18 [Meyuchasos 189]; Or Zarua 1:140; Meiri Megillah 21b; Teshuvas Riy Ben Palat, brought in Abudarham  Tefilos Chol; Binyamon Zev end of 169; Hagahos Rebbe Akiva Eiger 240:1; Ketzos Hachoshen C.M. 97; Tumim C.M. 97; Pesakim Uteshuvos 240:1; See Binyamon Zev 169; Sdei Chemed Mareches Brachos 16; Likkutei Sichos 36:91 footnote 8; Encyclopedia Talmudit Erech Kibud Av Vaeim Vol. 26 p. 374

[2] The reason: As a parent is able to forgive his honor, which would nullify the Mitzvah of honor. [Rashba ibid; Poskim ibid] As it is considered a Mitzvah between man and his fellow and a blessing is never recited by a mitzvah between man and his fellow as perhaps one’s friend will not desire the favor and hence no Mitzvah was performed. [See Chasam Sofer O.C. 54] Alternatively, as a blessing is only said on Mitzvos which are unique to the Jewish people, in contrast to the gentiles. [Binyamon Zev 169; See Aruch Hashulchan 240:4; Shevet Halevi 2:11] Alternatively, one only says a blessing over mitzvah’s that come from time to time and not commands that apply constantly. [Or Zarua ibid] Alternatively, one never says a blessing over a mitzvah that is understood and obligated by the mind. [Shem Aryeh O.C. 1; Taharas Mayim Mareches Chaf 35; Seridei Eish 2:46]