don't know if I can speak for my entire nation, but, I shall try to explain it for myself...
1) socks, as a project, travel well
2) socks knit up quickly
3) socks are a good place to experiment with a new stitch pattern, yarn fiber, etc, without an excessive investment of time or money
4) socks are universal, everyone needs them, everyone wears them
5) socks make you feel like a genius! the first time you tackle a short row heel, the first time you pick up the gusset stitches, the first kitchner toe you tackle, the first time you knit a small circumference in the round, you feel So SMART that you figured it out!
6) socks are impressive! When others see you knitting a pair of socks, they instantly admire your skill!
7) socks are gratifying! success breeds success. once you've successfully created your first pair of socks, you want to snowball that success into other projects, preferably more socks! the more successes under your belt, the more confidence you feel as a knitter.
8) socks are creative! you change your socks every day. why not use that as an opportunity to experiment with a new color, new lace texture, new SOMETHING that you have never tried before?!
Well, I could probably go on and on. For now, I hope I have done at least partial justice to the question of "why all the socks, you yanks?"