The function 'Add to Calendar' creates a .ics file in most cases, which are compatible with many calendar systems.
Some of the newer functionality we've added, includes integrations with Outlook for Mac, and Google Calendar!
Thanks for your continued involvement and feedback on these tools!
I apologize for the frustration.
All development work for our products has to be prioritized based upon many factors, unfortunately integration with Macs native calendar is not something that generates a lot of demand. In my experience the majority of our Mac users are using 3rd party applications like Outlook or Google Calendar, which is why we released plug-ins for those applications.