Why do we mention prayer by the number of rak'ahs and not by the number of prostrations?

In the Arabic language, prayer (ṣalāh) is referred to by various names, such as "Qur'an al-Fajr," "sajdatayn" (two prostrations), "subḥat al-ḍuḥā" (forenoon glorification), and "rakʿāt" (units of prayer). "Qur'an al-Fajr" is so named due to the length and audibility of its recitation, and some scholars have interpreted it as the Fajr prayer. As for "sajdatayn," it refers to two rakʿāt (units). "Subḥat al-ḍuḥā" is so named because of the glorification (tasbīḥ) it contains. There is no issue with these appellations, but the most common term is "rakʿah," which encompasses standing, recitation, bowing, and prostration.