The wear and tear on the transmitters from signing on and off every night would cost more.

I can name multiple stations that would sign off at Midnight but stopped because it got to the point where the transmitter wouldn't turn back on.

It used to make sense when the FCC required someone to be on staff 24 hours a day. In other cases some stations needed someone to run the board 24 hours a day. A few still do. Now computers can handle it.