I suspect that the main reason it the complexity which, in turn, means they'd cost more than most would be willing to pay. To customize an unlined FRN model would, at the least, require fabrication of a new backspacer. If you wanted liners, they too would need to be fabricated. The lined ones use nested liners, so the new scales would either need to be inletted to fit the existing liners, full liners fabricated, or you'd have to go without liners.