So I can say this:
All of us eat.
Using "us", an object pronoun, and saying that everyone in a group eats. I can also say similar things like
All of them eat.
Which uses "them", also an object pronoun.
Why is an object pronoun used in these prases rather than a subject pronoun?
It's because "of" is a preposition. The object of a preposition needs to be in the objective case. (E.g.: "me", "him", "her", "us"; not "I", "he", "she", or "we", etc.)
The true subjects of your example sentences are "all". The pronouns you are asking about are just parts of prepositional phrases modifying "all".
