Maybe depends on the countries.
This is the key point.
My country has run several confiscations of guns, and scrapping by melting. Even some legal guns without CIC proof have been confiscated and scrapped.
Post-war, guns stockpiles are either sold for export or scrapped. This left no spare parts in circulation
We must have all guns proved by CIC, and we dont have CIC proof house in Country.
This leaves gunsmith workshops without the option to build a rifle from an action, or even to change the barrel.
Those who do must go to Austria for a proof test, and it is either too expensive (with entire trip, and gunsmith work hours) or they just do not bother. To buy action, you need all paperwork and licensing like buying a new rifle. So, all that trouble, just to get action, and then to start building a rifle.
Just to give idea on legalities: every serialized gun part is considered "a gun". Barrel. Receiver. Bolt.
It must be recorded separately and licensed separately, if bought separately. Nobody can take the trouble to collect spares, to build a rifle from parts. On the other hand, with one license you can buy a complete gun assembled.
So, a modern hunter generally has the option to buy a new factory rifle or a used one with CIC proof test.