Like everyone else has said, it's because they're the biggest bike manufacturer in the world, and the economy of scale comes into play. Also, and this is from personal experience, they seem to make entry to mid-level bikes that have really excellent frames but mediocre wheelsets, preferring to make their own wheels most of the time.
If you're interested in competitively priced bicycles from huge Taiwanese manufacturers and do not live in North America, you can always check out Merida as well. Their bikes are very comparable to Giant bikes in terms of market competitiveness and they own a 49% stake in Specialized, hence their absence from the North American market.