Why are no other verses or surahs recited after al-Fatiha in the third and fourth units (rak‘ahs) of obligatory prayers?

In the Qur’an, acts of worship are commanded without specifying their details. The manner and specifics of performing all acts of worship—whether obligatory, necessary, or voluntary—have been determined by the Prophet Muhammad (peace be upon him). He demonstrated how many units (rak‘ahs) each prayer consists of, where and how to recite, remember, glorify, praise, or supplicate, as well as how and how many times to perform bowing (ruku‘) and prostration (sujud). He said, “Pray as you have seen me pray.” (Bukhari, Adhan, 18 [631]). In other words, we are responsible for performing acts of worship exactly as they have been prescribed.

The fact that no surah is recited after al-Fatiha in the last two units of obligatory prayers should be understood within this framework. However, if a surah is recited after al-Fatiha in the last two units of obligatory prayers, it does not invalidate the prayer. According to the accepted view in the Hanafi school, it does not even require the prostration of forgetfulness (sajdat al-sahw). (Ibn Nujaym, al-Bahr, 2/102; Halabi, al-Saghir, 175)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu