Why do Muslims not unite in fasting, even though the Ramadan crescent is one, taking into account that the disunity of Muslims in fasting in the past was excusable due to the absence of media?

The primary reason for the difference in the start of fasting between countries is the variation in moon sighting locations (matali' al-ahilla), which is a fact known through sensation and intellect. It is not possible to oblige Muslims to fast at the same time, as this would require some to fast before sighting the crescent moon. Sheikh Ibn 'Uthaymeen clarified that unifying the Ummah in fasting and linking the moon sightings to Mecca is astronomically impossible. Thus, each country has its own ruling, based on the Almighty’s saying: ﴿So whoever among you sights the crescent moon of the month, he must fast﴾ [Quran 2:185], and the Prophet’s (peace be upon him) saying: «Fast when you see it (the crescent moon), and break your fast when you see it.»

Scholars have agreed that astronomical calculations are not to be considered for establishing the legal sighting of the crescent moon. The difference in fasting times due to the variation in moon sightings is a permissible difference, and each Muslim country has the right to choose what it deems appropriate.