Why most of Americans don't buy American cars like chevy or ford? There are lots of Japanese cars instead?

The Cadillac Escalade EXT and SRX, Chevrolet Avalanche, Chevrolet Aveo, Chevrolet HHR, Chevrolet Silverado and Silverado Hybrid, Dodge Journey, Dodge Ram 2500, Dodge Ram 3500, Dodge Ram 4500, Dodge Ram 5500, Ford Fiesta, Ford Fusion and Fusion Hybrid, GMC Sierra and Sierra Hybrid, GMC Sierra Crew Cab, Honda CR-V, Lincoln MKZ and MKZ Hybrid, Nissan Sentra, Nissan Versa, Volkswagen Jetta and Volkswagen Jetta SportWagen are all assembled in Mexico. Toyota Tacomas are assembled in both Mexico and the United States.
GM to Import Chinese-Made Buick SUV

This isn't 1982. It's not that cut and dry.
The Toyota Camry is more 'American' than any GM or Ford.

Actually most of the Japanese cars are made in the US, while many of the American brands are made in Mexico or Canada.

Because most foreign made cars are better. Simple. Japanese and Korean cars are better than American. European cars are not as good as Japanese or Korean, but better than most American. American car makers just don't seem to get it. Too bad.

Build quality of Japanese cars "overall" is higher. Also typically more fuel efficient.

They generally are not as good.

But I drive a 94 Buick.

I like being frugal.

The US car industry did not see the oil crisis coming. It also did not see the consumer desire for better quality car which handled better. Hence they ignored the imports of foreign cars which they saw as just a joke. By the time they woke up the foreign car market was large and they were playing catch up.
A WRX impretza will out perform and out handle most US made cars. This is all off a 2.5 litre engine.

Detroit paid the price. The US manufacturers are now making a lot of cars abroad.
Several foreign makers now have plants in the US but not in traditional car making places.

Lesson. Ignore your competition at your peril.

Look at Consumer Reports and see what the quality ratings are for most cars. In the annual book they list cars to avoid. 80% are American models. Poor seats, bad transmissions, rattles, and bad mileage come up a lot.

Both Chevy and Ford have most of their parts made overseas.