Why master user does not have all privileges on all tables in RDS postgres?

Question

In RDS postgres, I have a user/role named master_user with rds_superuser permission.

1. I created a new db user user1 using the master username/password.

2. I created a db table named test_table using the new user user1.

3. I created a new db user user2 using the master username/password;

4. I tried to grant all privileges on all tables for user2 while using master username/password. However, I got errors like this:

postgres=> GRANT ALL ON ALL TABLES IN SCHEMA public TO user2;
ERROR:  permission denied for relation test_table

Why I still got permission denied error while using the master username&password? What's the workaround here?