It's just the way LED bulb technology is as they are designed to turn on and off very quickly. In most cases it's not perceptive by the human eye, but for some it is. Most are 60Hz as they run off 60Hz AC. (reminds me of the 60Hz refresh rate of an old CRT monitor) I haven't been able to find any that are twice as bright at 120Hz. These are supposedly flicker free, but they still flicker... just at a much faster rate. As BigBlarg
mentioned you could use a full-wave bridge rectifier at the socket end of the strand to get 120Hz.
The above link I found quite interesting.--
Remember that cool hidden "Graffiti Wall" here on BBR? After the name change I became the "owner", so to speak as it became: Dustyn's Wall »[Serious] RIP