Why don't Americans use the metric system?

I have been in the US for more than 7 years. I have gotten used to many things in the US, but one of the things I am still not used to is the measurement system here.

All of the countries except three countries in the world are using the metric system. The US is one of them. The US is the only industrialized country where the metric system is not the official system of units.
I still have no idea how big, how long, or how hot something is... :(

Recently, I was shocked to realize that the volume of 'one cup' is different between Japan and the US. One cup is 200 ml in Japan, but 240 ml (8oz) in the US. I have been cooking using an American measuring cup for Japanese recipes. Now I understand why my dishes failed...