I know what you are thinking. They are probably on cheap bicycles where a set of lights might cost more than the bike itself. Well that is true for some but I have seen quite a few riders on expensive road bikes, hybrids and mountain bikes with no lights whatsoever.
My commute to work this morning was made more interesting by thick fog to go with the darkness. To my amazement I saw a few riders with nothing. It was much the same story on the way home. In fact I regularly see riders without lights. Some are stupid enough to wear black with not even the smallest amount of reflective material. This is just plain stupid.
I had a quick look at bike lights and you can get a very good set of front and back battery lights that would withstand the rigours of a daily commute for £9.99. There really isn't any excuse.
Here in London there are a sizeable number of cyclists who run their lights during the day - I am one of these - and although many would deem this unnecessary, for me at least it makes perfect sense.