I lived IN Boston for a while, and actually rode my bike to work during the summer (didn't own a car) and just let everyone suffer through my sweat. But I couldn't do it in the winter (yes I know some people do). Aside from the physical cold/misery, the snow and ice made many roads extremely dangerous. The last few places I've lived were in the suburbs, commuting to other suburbs (luckily I've been avoiding the worst city traffic). The distances were too great to bike (unless you like a 3+ hour commute). shrug. Its great if you live somewhere that biking works for you, but don't expect everyone everywhere can just give up their car, jump on a bike, and carry on with life...
My main point is that a lot of people assume that biking is the worse choice for them without actually knowing.
I have noticed that most people I know who have tried bicycle commuting and gave it up didn't really give it a fair shake. They used substandard or the wrong type of bikes, didn't use the accessories and clothing that make it feasible (rain gear, panniers, trailers, etc), and so forth. They just grabbed their old bike out of their garage and hopped on. That can work, but it's not usually the best experience.
What? How do you figure? My observation is that most people absolutely can (and many would seriously benefit from it).
Maybe not for every situation, but for the rare ones where a bike won't do, a cab will.