Judging by traffic on the other side of the highway, I'm guess it's because I'm commuting the opposite direction of most people. If you have parking, and traffic isn't terrible where/when you're going, the bicycle is usually slower.
But, on the other hand, riding a bicycle is so much more _fun_ than driving a car. Riding a motorcycle can also be fun, but still always doesn't beat a bicycle for me.
The most astonishing thing for me was the amount of energy I got from it. I used to be quite apathetic during the day, with intense sleepiness in the early afternoons, and now I feel awake during the day. I cannot overstate how beneficial this is
Took a substantial pay cut in large part to avoid the soul killing commute and to be able to bicycle to work (~9 mi one way). Probably the single biggest life event in the last five years (after having kids). Used to do it daily but recently stopped after having to sort out logistics with school drop-offs, so I'm shopping for an ExtraCycle to address that, then back on the wagon!
I'm pretty sure I solve some of my biggest problems while I let my mind wander on my ride home. It can be intimidating at first so I don't recommend immediately hopping on a bike and riding in traffic, but take a few weekends to get a feel for the bike of your choosing (lots of great commuter bikes out there) and then slowly get used to riding with cars zooming by you.
I've been doing this for a few years now and have never had more energy at work.