Recently, when looking for IIS logs on the WFE servers of one of our environments, was literally dumbfounded by the absence of ANY IIS logs!

Checked IIS and logging was enabled.

Security for the folders looked good.

Confirmed that a scheduled task to delete IIS log files older than 30 days was configured correctly.

What could have caused this?

Did some research and discovered that W3C Logging service was not running. That service is the lynchpin for IIS logging.
Wow. How could I not know this?

One other thing: if you created the folder for IIS logs before setting up logging with IIS, this could be a problem. It appears to be a permissions-related problem.

Better to create the folder through the IIS Manager UI. This will assign the correct permissions that allow IIS to write files to that folder and subfolders.