Giants in Eberron are native to the continent of Xen'drik. The dragons made contact with the primitive races of giants 60,000 years ago, teaching them the arcane arts and watching them build their own civilization. The giants were masters of the arcane arts, but much of their empire was built on the backs of slave labor, more specifically elf slaves. For, the giants took the primitive elven race and kept them at their sides as slaves.
40,000 years ago, the Quori invaded the plane of Eberron from Dal Quor, the plane of dreams. The giants used their tremendous magical powers to sunder Dal Quor from Eberron, severing the invaders from their prize, but also causing terrific devastation to Xen'drik and ultimately, their own civilization. The dragons punished the giants by destroying their civilization and the elves used the opportunity of another war to rise up and throw off the yoke of slavery, bringing an end to the giants' 20,000 year rule.
Today, the giants are mostly primitive savages, living among the jungle ruins of their own once great civilization.