Because by giving the adhan while standing, even a person far from the mu’adhdhin can hear the adhan well, whereas this benefit cannot be achieved by giving the adhan while sitting.

Imam Abu Bakr (may Allah have mercy on him) says that in the narration of Sayyiduna Ibn Umar (may Allah be pleased with them both), the Messenger of Allah (peace and blessings be upon him) said: "O Bilal, stand up and call the adhan for the prayer."