Why "useradd -g" do not update /etc/group ?

Issue

• Why "useradd -g" do not update /etc/group file ?

Add a new user by following command line:

# useradd -u 1000 user1

You can find in /etc/group, a new record of "user1:x:1000" is created.

Add a new group and a new user by following command lines:

# groupadd -g 1000 group1

# useradd -u 1000 -g group1 user1

There is no error but you can not find any information about user1 in /etc/group.