There is one other reason.

There is a large market for the parts because the rifle is so easy to assemble and disassemble. For a rifle like the Remington 700 folks will maybe buy used stocks, there is a small market for used barrels but most people going to the trouble of putting a barrel on a 700 will buy a new one. By the time you pay a gunsmith to re-barrel one you might as well spend another couple hundred and put a real nice barrel on it. And unless they are having a custom rifle built usually don't change bolt head size to accommodate caliber.

Just like with AR15's any one can take a Savage 10/110 apart with a few simple tools so you have a greater number of people willing to buy the parts and mess around with the rifle. It's so easy to change parts that people are willing to buy a used or take off barrel to try out a new caliber.

So it's Business 100 Supply and Demand. There are people that want the parts creating the demand.