You can just lower the current. Not everyone does because it generally requires more expensive components, e.g. inductors. There is a threshold voltage ("forwar

You can just lower the current. Not everyone does because it generally requires more expensive components, e.g. inductors. There is a threshold voltage ("forward voltage") needed for LEDs to turn on but there's no threshold for minimum radiant flux. LEDs are actually more efficient at low current (although this might be counteracted by greater losses in the power supply).