No output from ros2 topic hz

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When I run ros2 topic hz for certain topics, there will be no output and the terminal will lock.

For example, I have launched turtlebot3_world.launch.py and run teleop_keyboard, and also am running rqt_graph (I'm not sure if all these will be relevant but I'm just stating the current circumstances). Then when I run

user@ubuntu:~/ros2_ws$ ros2 topic hz /scan
average rate: 4.995
    min: 0.199s max: 0.201s std dev: 0.00046s window: 6
average rate: 4.995
    min: 0.199s max: 0.201s std dev: 0.00060s window: 11

And it goes on outputting the average publish rate. This happens for /clock, /cmd_vel, /imu, /joint_states, /odom, /tf, /tf_static.

However, for /parameter_events, /robot_description, /rosout, this happens:

user@ubuntu:~/ros2_ws$ ros2 topic hz /rosout

^Cuser@ubuntu:~/ros2_ws$

The terminal remains locked and no average publishing rate is outputted (I had to Ctrl-C the process).

May I know why this is so? Is it because it is not publishing any msgs at all?