If you don't know the history of work on the engine it's possible a previous owner removed the coolant thermostat.
Or, it may have failed in the open position. If you can see a good amount of coolant moving thru the radiator when the engine is cold and the thermostat should be closed, that would be a meaningful clue.
This is a '57 T-Bird engine photo but the thermostat is in the same place.