Why is it not possible to restore deleted plugins from any track that has automation?

If you remove any plugin from a track that has automation, restoring it deletes all automation data.

Why?

it’s the removal of the plugin that deletes the automation data, not the restore. The history (and the mix console snapshots) simply don’t store automation data and cannot recall it.

Why does removing a plugin which doesn’t have any automation tied to it affects automation of other plugins?

It makes on sense.