Free: Contests & Raffles.
Recoil management and options like different pistol grips, adjustable stocks and arm guards which make getting into a shooting position more natural. And, the Zombie problem. You never know.
ARs are really just Legos for big boys and girls
I think also that someone has the opportunity to build a top long distance MSR for about half the price it would cost to have a standard rifle built to the same accuracy capabilities and same comfort.