Giant's carbon technology (r/d, production methods, etc.) is second to absolutely no one's.
As others have pointed out, economies of scale come into play as well, in two senses. First, Giant produces massive numbers of frames, not only for their own brand but for many others, including many of the 'cachet' names. For example, the Treks that have been suggested to you as alternatives will have frames made by ... Giant, for Trek.
Second, as pointed out also here, economies of scale mean that Giant gets preferential supplier pricing from the likes of Shimano for components.
The result: if you like a particular Giant bike, and it fits you, you lose absolutely nothing at all by going with that rather than a competitor's equivalent, and you will typically pay significantly less.
Disclaimer: I have only a very small dog in this fight. My two main bikes are Specialized, but I do own an older (2005) Giant mtb that I still use throughout the summer. It has been and still is a great bike. I like Specialized, but wouldn't hesitate to buy another Giant bike if it was right for me.