It appears that the phenomenpn is not limited to the US - I observed very similar thing in Eastern Europe whose school system still rests on the institutional foundations built under state socialism.
Methinks the culprit is the changes in society that create fewer and fewer opportunities for spontaneous face to face interaction among people. Wits, like any other human skill is "use it or lose it." They stay "sharp" only when they are routinely used - and the opportunity for such use arises from spontaneous interaction. But such spontaneous interaction is on decline here in the US and elsewhere. Here are a few culprits behin this process.
Entertainment industry. In the "good old days" :) people did not passively sit in front of an entertainment machine waiting to be stimulated - they had no other choice but to entertain themselves by conversation, telling stories, jokes, doing interesting things - the more active and involved people were - the more intersting their company, their get togethers etc. Such interaction greatly stimulates cognitive skills - you have to think quickly to say something interesting or funny. But with the development of entertainment industry, entertainment machines being present in every home, people have fewer opportunities to use their more creative cognitive skills.
Disappearance of natural communities and public spaces. Human interaction needs physical proximity because such proximity provides more opportunities for interaction, more channels for communication (verbal, nonverabla including sound, body language etc.), more modes of communication, such as "spot" exchange among strangers (e.g. people asking for direction, commenting on an event, etc.), casual exgange among stranger (e.g. conversation at a barber shop, laundry room, public transit, etc.), casual exchange amoung acquintances (such as intearction with neighbours), as well as the sahred environment that create the basis for common interest.
With the disappearance of nutural communities and public spaces, the opportunities for interaction disappear and the interaction that remains thends to be based on a single mode and channel - such as telephone conversation that is limited to verbal communication, "professional" exchange that is highly conventionalized and eliminates the variety of more subte forms, such body language, innuendos, olfactory cues etc. Moreover geaographical dispersion due to suburabanization and autmobilization, and the separation of work and private virtually destroy the community of interests. I sepnt most of my physical time at work - yet my interaction with others there is limited by conventions and rules of professionalism. After work, I am free to interact with anyone I want, but since I do not share the physical environment with those people, there is little common interest, and little to talk about - unless we of course we share the same cultural environment manufactured by the entertainment industry - teevee, sports, rock-and-roll and and assorted cultural products.
Changes in college culture. I do not think that schools and colleges ever required anything other than following rules and instructions. Just spit back at the teacher what you've been told, point, clink, no think. What changed, however, is the college informal culture. Some 20 years ago being original and different was cool. Standing out of the crowd was a sure way of gaining popularity, meeting new people, getting into interesting situations. Today, however, fitting in is cool. Anyone perceived as being outside the mainsteram is considered a weirdo and avoided - conventionality rules.
And since these social forces are not about to disappear, thing will most likely get worse, not better.
regards,
Wojtek