why does fitzgerald list all of gatsby's party guests

This is so we get a sense of the type of people who come to Gatsby's parties. There is a theme here. They are generally rich and have little interest in Gatsby himself (other than the mystery behind him). Fitzgerald generally describes them as self-absorbed and only interested in the excess of the party, "the people came like moths to the flame."

To just show how many people attend Gatsby parties and of what class is each from to get an idea of the people that show up and some are generally rich but like said before have little interest in Gatsby himself.