Have you studied Knights or LMT? They aren't just your run of the mill AR selling as a top-tier. These are not mil-spec rifles.
I don't own one and probably never will but understand quality and attention to detail when I see it.
There is a reason why Knights Armament AR's never hit the civilian market and when they do they are snatched up very fast. They cannot keep up with sales to Tier 1 operators in the military, why is that? Just snobbery?
The only reason I suggested those two was because "cost was no concern" of the OP. Otherwise they wouldn't have probably made my list because no common civilian really needs them IMHO; as there are a host of good AR's out there for 1/3 the cost.
I've never seen a Knights in the flesh. I have seen a LMT at my range in the hands of a off duty PSP officer (heck of a nice guy to boot!), and while the finish was perfect, and it looked good, it performed no better where it counted...down range. He and it, shot just as well as my Anderson build. He even joked about the cost himself...
I've also seen a few Daniel Defense at my range, and again, while nice, I saw nothing that justified the extremely high price.
One brand that is worth the cost..in my opinion, because they ACTUALLY do use better material, yet stick to the Mil Spec fitment...is JP Rifles. In my opinion, they are the real deal. I have one of their bolt carrier groups in one of my builds...and it is, indeed quite a few steps above Toolcraft and Microbest...but it also set me back $450 bucks, compared to a $150 Toolcraft. For chizniks and giggles, I even swapped out that carrier with my spare cheapo..just to see if the rifle shot better or worse. It didn't. Same holes. So..was the extra expense worth it? Depends on your point of view....
So..if Knights isn't Mil spec...then it's not really AR pattern, hence your answer as to why they would cost more. That puts them into the custom side of the equation...
Why they can't keep up with demand...well, that could be from a lot a factors. The cost would come from supply and demand...plus the selling point that Seal teams and special forces use them (and frankly..if any one REALLY needs a top notch custom weapon...it's those guys), which drives those in Meal Team six to lust after them..creating even more demand...which is where the snobbery comes in.
So..to sum it up...
Real world Dark ops operators= legit need for state of the art gear..
Meal Team Six= Willing to pay top dollar just because real operators use them...for reasons of ego and bragging rights....