I ride to work all the time too, but there I days I have to cage, such as if I have to carry something to work, not going home after work, golf/gym/grocery shoping etc...
There are TONS of people who do not ride to work that have bikes. Motorcycles on the roads are a minority. Motorcycle commuters are a minority inside of a minority. To get them to all do it, you have to have a "day". It makes the ordinary average guy feel part of a group, part of a cause, part of something more than just a day at work.
Sometimes I want to wear sandals (and/or shorts), and sandals and motorcycles don't mix.
Sometimes I work out in the morning, and am too sore to ride.
Sometimes I'd rather listen to my music than my engine (no iPod).
Sometimes I'm sunburnt, and the wind hurts (yes, even through the jacket).