Power density / irradiance intensity. That is the main difference. The power density of most grow lights is very poor. Usually less than 10 mW / cm2 a few centimeters from the light. Much lower than the 100-200mW / cm2 ranges emitting red light therapy devices.  This is a comment RedLightMan made about grow lights: the wide positioning of the LEDs, the wide beam angle etc. that you will find in these types of 'grow light' devices in general make them unsuitable for light therapy.