I did this during my quest to find a flicker-free LED light bulb. I found that basically all of them flicker. Some appear to alternate between banks of LEDs to reduce the dynamic range(?) of the flickering, but they all flicker. Home Depot has a rack of LED bulbs that are always turned on, so that's a good place to test many models at once.

My first thought was: why don't they just filter the output? My guess is that perhaps LEDs are more efficient when being driven full ON or OFF rather than with a steady DC.