The process to chrome the interior of the bolt carrier if and gas key, if nitrided, requires the surface that is to be chrome plated to be copper plated first, then nitrided, then said copper plating removed, and then chrome plated. Another method would be mechanical which would work for the bolt carrier itself, but not the gas key at all. Simply put, it makes no sense as said BCG would cost more to produce with no improved results.
May not make sense, bu then again, a lot of things don't, yet are done just to make an extra buck.
If the cost could be amortized over many, many units and the end-result give nothing but perceived-value that helps attract customers in an area of the field that is just getting more competitive by the day, I can see a bigger player doing some math, and going "huh...."
That said, I have no idea how much that much work would cost.