Because the Noble Prophet (peace be upon him) has stated that this is a righteous deed. And he also stated that if there was no fear of causing hardship to those who provide Zamzam, I too would have drawn water from the well along with them.

Sayyiduna Ibn Abbas (may Allah be pleased with both of them) narrates that the Messenger of Allah (peace be upon him) came to a water reservoir and asked for water. Sayyiduna Abbas (may Allah be pleased with him) said, "O Fadl, go to your mother and bring a drink for the Messenger of Allah (peace be upon him)." The Prophet (peace be upon him) said, "Give me this water to drink." Sayyiduna Abbas (may Allah be pleased with him) submitted, "O Messenger of Allah, people put their hands in this water." The Prophet (peace be upon him) said, "Give me this water to drink." He drank from it, then the Prophet (peace be upon him) came to the well of Zamzam, where the people of Banu Abdul Muttalib were drawing water and giving it to people to drink. The Prophet (peace be upon him) said, "Continue drawing water and giving it to people in this way, for you are doing a virtuous deed." Then he said, "If I were not afraid that you would be overpowered, I would also put the rope on my shoulder and draw water," and he pointed towards his shoulder. Imam Abu Bakr (may Allah have mercy on him) says that this statement is of the same kind about which we say that a gesture also serves as speech.