Derek Z. over on the iBOB list, opined that "I feel folks are in a rush to get anywhere - shopping (big one for sure), to buy gas or a gallon of milk...and especially to get home." That's certainly part of it....
"Hurry" has become a sacrament for Americans. As has total and institutionalized self-centeredness. Part A makes you feel important; part B lets you feel that everyone else is inferior.
And of course they want to get home and out of their cars. Unless you're in a sports car on a winding, empty road, driving is a complete bore. You're strapped in a box and looking at the world with blinders on, can't hear, are going too fast to appreciate the life around you (if there were any; a freeway is as sterile an environment as a prison), stopping to get out is cumbersome, and trying to keep going is frustrating. The other drivers out there are "in your way," but you know deep inside that you're in someone else's way.
They dump on cyclists because cyclists seem less powerful, in part, but more, I think, because cyclists are freer (Yes, they hate our freedom!), and mostly because we're identifiably different--the same impulse that underlies racial and ethnic prejudice. It isn't logical, of course--if driving were logical, we'd take the $60 billion US governments spend subsidizing driving and put it towards mass transit, which is not only more efficient but less work for the traveler.
The underfunding of mass transit leads us to another aspect of driving, that of purposely (if perhaps unconsciously) undermining community in the US. We have spent seventy years and billions upon billions of tax dollars ensuring that people not only travel isolated from each other in little tin boxes (as opposed to big ones such as buses and trains, where I can almost always find an interesting conversation if I want one), but also that they live in suburbs, well-separated from each other, in pseudo-neighborhoods where there are no public spaces (except streets, where you're expected to drive, not bike or walk or play or visit), and shop in big box stores distant from their homes where they won't meet neighbors and establish relationships wherethrough unmediated discourse can take place. ("Unmediated discourse," communication directly between two people, unvetted by corporate or government media.)
The internet has made a dent in this, but the pinstripes are trying to turn it into a big mall; there is a growing countermovement trying to keep the Internet a free and open forum. Search (on Google or Alta Vista or any other search engine--did you know there is more than one?) the terms "net neutrality"--important to know about if you value discussions such as we often have on boards and blogs and forums all over the 'Net.
Cyclists are seen, often rightly, as rebels of the countercultural sort (more like hippies than Wild Ones), whose simple existence questions the driver's acceptance of a social order that trammels his life in steel and disinformation.
Nevertheless, I see a lot of drivers who have become cyclists at least part time here in LA, which is a difficult place in which to ride transportationally. A significant number of those are taking on a more egalitarian worldview, helped along by places, like Orange 20 (one of our dealers), that specifically promote "urban egalitarian cycling." (In fact, a number of cyclists I know work in co-operative, non-hierarchical organizations.)
Using bikes to travel means at the least that you don't spend as much on fuel and cars, and can mean that you opt out of the rabid consumerism and Fox News worldview that are meant to define American culture and justify one's serf-like position in it. The false sense of power driving gives one is recompense for a trammeled and limited lifestyle.
So to most drivers, and especially to urban/suburban SUV drivers, a cyclist is more a rebuke than an impediment. I often (almost every day) am passed and repassed by hurrying drivers with a great roar of engine and tires, only to catch up with them repeatedly as they get stuck behind masses of cars just like theirs. Sometimes someone will roar around me to gain twenty feet on my position at time of pass, and end up behind a long line of cars waiting at a light, which I then pass. A light that was brightly and visibly red when he initiated his pass to gain that twenty feet.
This is not rational behavior. Basically, drivers in the US induce a voluntary insanity to justify driving everywhere all the time. They don't really need to go fast, they don't really need to carry much 99% of the time. They need to feel powerful, and to feel accepted by a culture that overwhelms them. The car is sold to them as the means to these sensations. It fails, and they are frustrated. Yet they try the same thing again and again, climbing into the cab and getting stuck in traffic. The magic always fails, as magic always does. And you gotta blame somebody for breaking the spell.
I suggest you read Andre Gorz's brilliant analysis of car culture, The Social Ideology of the Motorcar.
And there's a whole page of links to articles by me and others on this matter, including everything from rants to descriptions of community-friendly community design by prominent architects, in Bicycle Fixation's Sustainability index.