Giant is Taiwanese-based. They make nearly 90% of the bicycle frames in the world, including most other brands'. Companies that I know of using their frames include Specialized, Haro and a host of others. Not that it's a bad thing, they make nice stuff.

I think much of the reason you're seeing so many treks is the "Lance" factor. His team was/is sponsored by trek and as a result has the most exposure over the past few years.