Why does Japan have 8 car companies, but America just has a "Big 3"?

Solid question, I'm going to say that the Japanese auto industry never consolidated due in part to the keiretsu system. For example the Mitsubishi group remains committed to being in the car business, so Mitsubishi Motors is not for sale. Some of the smaller companies like Mazda and Nissan now belong to larger companies and that is why they are still around. Daihatsu has its niche with the small cars.