Why don't Muslims unite in determining the beginning and end of the month of Ramadan, and how can they achieve this unity?

Muslims come together for fasting and breaking the fast in the following ways: Either scholars disregard reliance on calculation and act upon sighting or completing the number, as indicated by the authentic hadiths. Or they adhere to relying on the confirmation of the sighting in any Islamic country that acts according to Allah's law. So, whenever the sighting of the crescent moon is confirmed by that country, they follow it, acting upon the saying of the Prophet, peace and blessings be upon him: "Fast when you see it (the crescent moon), and break your fast when you see it. If it is obscured from you, then complete the number."