Why is the age of Islam not calculated from the beginning of prophethood instead of the Hijra, by adding the thirteen years before the Hijra to the current date?

The years of the Prophet's call in Mecca before the Hijrah are a great part of the life of Islam. However, the Companions, during the reign of Umar ibn al-Khattab, may Allah be pleased with him, agreed to adopt the Hijrah as the beginning of the Islamic calendar, because it represents the true date of the establishment of the Islamic state and the emergence of its features. The Companions chose the Hijrah for several reasons, including the clarity of its date, unlike the birth or the prophetic mission, and to avoid the date of death due to the sadness it entails. Moreover, the beginning of the resolve for Hijrah was in Muharram, which made it the first month of the Hijri year. This temporal designation does not mean neglecting the early years of the call; rather, it is for organizing transactions and timings, as the true beginning of Islam preceded the Hijrah.