Q. Why does ending with a good topic not apply to one ending a daily daf or any shiur on a sefer?

A. Actually, although it is not widely known, it does apply to all areas and acts of Torah study, as the Talmud (Brochos 31a) teaches that he prophets ended their words with desirable and correct termination (See Tosafos ibid.).
Minhagei Chassam Sofer mentions that he was always extremely careful to finish any conversation. even sichas chulin, wuth a good ending.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a