I noticed it right away a few years ago when i started riding alot again. I just assumed the following

1, the ride of the carbon fiber frame was just too rough for anyone to let go of the handlebars.

2, since the sales person told them "with these brifters you never need to move your hands to shift or brake" everyone is afraid to let go of the bars

3, everyone is just too busy trying to best there last personal best so thay can make the Olympic team

most importantly I ride alot around So jersey in the Lumberton, Medford and Mt laurel ares and I just assumed everyon thought they were better than everyone else (you should see all the lexuses {Lexi?** infinity, and excalades around here) and therefore there was no ned to recognize any other person.

personally I always way, I am not big on the whole head nod thing. I think I get a response about 50% of the time