Even as an Admin, I only have access to those Calendars that I've created or someone has shared with me.
It would be great if ALL calendars were accessible for Admins in a list similar to reports.
I understand the way calendars are currently accessed, with the calendar names across the top of the page like tabs you wouldn't want access to every single calendar in the system. It could potentially be impossible to see the calendar itself because there might be so many rows of calendar names.
But I'm often asked to troubleshoot things that aren't showing on a particular calendar, which is impossible to do without access to that calendar. This sometimes means guessing who may have created that calendar, logging in as them and sharing it with myself.