ROS is verrrrrrry opinionated and a pain to set up in an orthogonal way to other ROS installs. It's also tied very heavily to ubuntu/debian. Putting it all into containers makes many things so much easier (it makes a few things harder, or at least it did on ros1, hopefully that's been ironed out in ros2)

I have no idea what you mean by "just linking the filesystem".

Not having to screw with your system's dependecies and spend time making sure everything is aligned with the expected environment.

Ansible like tools could help you make the setup repeatable, but being containerized also avoids all the processor architecture and system libraries gotchas.

I don't want to install anything on my system, want to keep clean and as minimal as possible, containers full fill my OCD in this regard, I run most app's now in containers too, it is best linux setup I ever had in my life :)