The issue isn't with Zoom, it's with Microsoft's own .ICS files. New Outlook doesn't open them by default. You need to Import each .ICS file individually by following these steps.
- Download the .ICS to your computer.
- Open New Outlook and go to the Calendar view.
- Drag & drop the .ICS file from step 1 onto the Calendar.
An Add calendar window opens on the Upload from file page. - Select your calendar from the list, then select Import.
Another option (but it's more steps) instead of step 3 above, click the Add calendar button, select Upload from file, then Browse for the file from step 1, then continue to step 4.