I believe the reason is simple. Folks paid good money for a product. They should expect it to be everything that it is supposed to be. Way too many gun shops spout the "just as good as" mantra. To them it's all about profit. Add to that the complete lack of any real knowledge of the part of the staff.
Often these brands due just fine in peoples hands because most folks just don't shoot enough or in a compressed time period to make difference. It's only when these brands are placed in a real world use environment, such as one simulated in a high round count class that the differences become apparent.
As has already been stated, with the cost of making them right (not including qualified labor) you could have purchased a tier one brand to begin with. Unfortunately that's one thing you will never hear in most gun shops. Colt's/LMT's/BCM's/Noveske's/Larue's/Etc. are just not as readily available and/or the initial investment for the purchaser is higher so there's less chance of making a sale.

