Whom needs to get hooked-up?
The guy who builds you a custom or customized AR knows you will get known distance shooting, sighters & re-shoots. Don't exspect any of Bin Linden's friends to be so generous.
The demand for a stock Colt is higher, new or used, than AR clone custom anything. There are more uses for a stock Colt AR, hence more customers looking for one.
Customer builders can make a AR as reliable as a stock Colt, but not more reliable than a stock Colt. A custom AR that is as reliable as a stock Colt costs more than a stock Colt.
Since the majority of users cannot shoot up to the intrinsic accuracy of a stock Colt, especially in field use, the issue of accuracy isn't a big factor.