Lady Aisha (may Allah be pleased with her) narrates that I and the Messenger of Allah (peace and blessings be upon him) used to perform ablution from the same vessel.
Because if a specific amount of water had been prescribed for ablution, such that using less or more than that amount would not be permissible, then it would also be impermissible for two people or a group to perform ablution together from one vessel. This is because it is certain that when they perform ablution together from one vessel, some people will use more water and some will use less.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب ذكر الماء الذي لا ينجس، والذي ينجس إذا خالطته نجاسة / 119