Why can't I create a user with primary GID 100?

In /etc/group there is a group users with numerical group ID 100. When I install RHEL 7,8, or 9 it **does not** allow me to create the initial user with a primary group ID of 100. The GUI always snaps back to 1000 which creates a new group having the username as a new group name. I do not like that. Why does the RHEL installer (and presumably any other currently), not allow you to set that user's primary GID at install time to gid=100? I have the assumption it is ok to have some ~50+ local users on the system defined in /etc/passwd all having a gid of 100 users. Is there any downside to this? What is the convention and history of gid 100 *users* ? Does users having a low gid (100) versus 1000 and above pose any kind of problem?