They go steady as well. I just reckon that seeing as I don't actually need them to see, with street lights everywhere I cycle, they are there for me to be seen. A flashing light feels more visible.
I commute in London and have rear and front lights that flash irregularly. I read somewhere that in cities, amongst all the light pollution everywhere, a steady light could get lost. You're more likely to see a flashing light. Made sense to me.