Before I understood Docker, I used to have HA installed directly on bare metal side by side with other “desktop” apps.

To be able to access devices, HA needs many different OS-level configurations (users, startup, binding serial ports, and much more I don’t have a clue about). It was a giant mess. The bare OS configuration was polluted with HA configurations. Worse, on updating HA, not only did these configurations change, the installation of HA changed enough that every update would break HA and even the bare OS would break in some ways because of configuration conflicts.

Could this be managed properly through long term migration? Yeah, probably, but this is probably a ton of work, for which a purpose-built solution already exists: Docker. Between that and the extra layer of security afforded by dedicating an OS to HA (bare metal or virtualized), discouraging the installation of HA in a non-dedicated environment was a no brainer.