It could be that it just never came up. This is the basis of the long Magen Avraham and Mishnah Berurah (O.C. 156) which list many Torah concepts which were not codified by every classic Halachic source, such as the Rambam and Shulchan Aruch. The Gemara mentions this concept five times without argument (see Yefei Einayim Avoda Zara 10b), clearly implying it is Halachah. If this does not satisfy some readers, you can see it applied in a Halachic context by the Teshuvos Beis Yehudah (Y.D. 17).

All the best,

Yaakov Montrose