macOS Calendar default not available
Solution 1:
I also had this and was frustrated with it (enough to open an account here just to reply). I only have one CalDAV account as I use IMAP for mail. My work around was to turn iCloud calendar on then minimise it in the left side as well as untick it. Events now get added to my personal account and I can set it as my default while the iCloud calendar sits empty in the background.
I have opened a case with Apple and they have confirmed the bug and submitted it to engineering for review/fixing.