There are a bunch of reasons. I can speak for myself. A) my gun store didn't have the complete fun I wanted. B) I figured I would learn more "building" my own. C) wanted to customize my own gun and make it perfect for me, and include everything I want in a rifle.
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Exactly. They did not have the lower I wanted with an upper, I want to build mine from the ground up, and I want to get it just right.