IMHO: Hand-fitting a barreled/action, barrel-bands, butt-swivel or sling-bar, and butt-plate to a stock blank is all very labor intensive and requires skill and lots of time.

Final shaping and finishing a gun stock also is time consuming and requires skill and care.

A 'drop in' Finished Stock would be very expensive and require some compromises (generous tolerances) to be 'Drop In'.

That is why loose, original, serviceable, attractive Krag stocks command so much money. (Also the same for other military stocks).