It is a traditional practice. What does happen from time to time as the oil hardens over a couple of years it will sink more into the pores of the wood giving the impression of not having been grainfilled.

Why no oil, time pressure and the availability of modern finishes. The finish in the first pictures look to me like an epoxy oil finish, one coat, 24hr hardening, checkering done and it is waterproof(80% iirc), unlike the traditional oil finish.

In the olden days there wasn't any exhibition circassian walnut available afaik, most makers used French or even English walnut and they can never get near to the exhibition grade of Circassian walnut. Same goes for engraving how many old guns have you seen with gamescene engravings? It was Ken Hunt who started the elaborous engraving on guns afaik. Most had standard scroll on them, no bulino, deep carving of animals etc... Expectations of a gun have gone up over the years. (I'm talking pre 1960's)