I would like to add we live in a 24-hour society.

People can shop, bank>>>on their computers, iPhones, etc., deliver food, drive a taxi, Uber, etc.

The world no longer stops after 10 PM.

There is an audience for radio after midnight. Some other reasons have already been stated within this thread.

Lastly, loyal radio listeners to a station aren't in the habit of tolerating double formats.

A station may dump their format overnight and play music which serves a different audience. When the sunrises, it's back to the original format.

If I'm driving to work and listening to WXRT-FM, I'd expect to hear the same format ten hours later on the drive home. The exception would be specialty programming, and it's advertised in advance.

I recall suburban WYSY-FM 107.9 which played adult hits during the day and hard rock at night. Chicago had enough stations serving that audience with a further reach from the Sears Tower or Hancock.