Marketing. Perceived value.
The parts are made by just a couple of suppliers.
All AR's are just as good as every other one. No matter what the hype and marketing.
Your $600 AR will be just as accurate, reliable, and will last just as long as the $2000 AR.
This^^^ IMO the price difference just isn't worth the difference. I have never owned a super high dollar factory gun but I have handled and shot them quite a bit. While they were indeed nice high quality pieces that I could never afford, I just couldn't see the reasoning behind paying their high cost. They will not do anything (performance wise) that my self built sub 1k AR's will do. But they sure are purdy.