You might as well ask why Americans hate Southerners or Southern Californians or Midwesterners. Or why hipsters from Portland and NY are made fun of or why Americans hate Canada and Mexico and Europe and East Asians and of course Central Asians and Semitic people. We're a nation of haters.
I don't personally know a single American that hates Canada or Mexico. In fact, I would say the opinion of Canada is between neutral and quite positive. They've just about never done anything to wrong us, have been a good border partner, and an amazing trade + energy partner.
I also don't know a single person that hates Silicon Valley. Most Americans, in my opinion, are clueless about Silicon Valley, outside of knowing that there are technology companies there. Why? Because the people / culture and politics of Silicon Valley don't matter to them and their day to day lives.
My post was partially tongue in cheek. American's don't universally hate Canada or Mexico or Silicon Valley. In the right context though many of them might sneer at Silicon Valley (or for many at least the ultra-liberal bay area). Ask most Canadians living in the states and you will hear innumerable sad stories of being the butt of not very funny poutine and maple syrup jokes.
