edit: so many replies!
I guess I stand corrected, although the more I think about it, the less I'm surprised. I live in Japan, obviously known for its public transport. Interesting to note though once I travel abroad, thanks for the replies everyone :-)
If I had to prioritise the reasons I cycle to work, speed and cost would come above the other things you mention - although they're nice bonuses.
Driving means: walk 1-3 blocks to my car, drive it in slow(and extremely variable based on time) traffic, circle the block a half dozen times to find a parking space, probably having to pay.
Biking on the other hand means: I walk outside, bike where I'm going at the same speed every day, and lock up when I get there.
I've frequently beaten my friends when going places in the city and I have my bike and don't wanna leave it behind so I bike and they cab/drive.
For reference, this is South West London to West London (6 miles).
There are a few factors to consider. There are places you can go by bike that aren't always available to cars (cycle paths through parks, for example). You'll get from the exact starting point to exact finishing point. And general congestion will take care of the rest.
In a slightly different way, where I am now, the fastest way to get to work is to cycle 20 minutes to a more convenient train station and take the train in. The alternative is a) much less reliable and b) takes an extra 45 minutes (minimum).
A light bike with a good set of gears helps a lot.
And generally speaking, bikes are just like any other non-public transit method: the purpose is to get from point A to point B as quickly as possible, fsck everything and everyone else.