While following the ROS2 humble tutorial, I faced the same challege. I tried running the sudo rosdep init followed by rosdep update severally for days but there was no success.
This work-around helped. I downloaded the base.yaml,osx-homebrew.yaml, python.yaml and ruby.yaml files directly from github. And copied them directly to the /etc/ros/rosdep/sources.list.d/ directory.
In the /etc/ros/rosdep/sources.list.d/20-default.list file, I changed the github links to this:
yaml file:///etc/ros/rosdep/sources.list.d/***.yaml
(replace *** with the names of each of the .yaml filenames)
finally, I ran the rosdep update command and it worked.