Giants are nephilim, they were here and made slaves of humans to mine and......to eat.

Something happened that they didnt expect, an anomaly if you will, and that was the slaves turning against them.

There are caves in the US where some giants fled to, and died in them because the slaves smoked them out to kill, or sealed them so the giants died inside the caves.

The giants are the nemisis of human kind and some were buried as seen form burial sites the world over.