They didn't lay 300 eggs per year in those olden days though. The demands on their bodies were much lower. They laid eggs to reproduce and ate the shells after the chicks hatched, and that was that. It was only after humans domesticated them, that they started laying outside of the reproductive season, continuously, day after day, and the demands on their bodies increased. Today's hens, especially the popular production breeds, would not be able to keep this up without help from humans. So it's not fair or realistic to compare them to their wild counterparts pre-domestication.
If they don't eat much else besides their poultry feed, then they don't need a lot of grit - poultry feed itself is water soluble and doesn't need grit to be digested. Whatever they find while free ranging that needs grinding, they can probably grind with rock they find out and about, or the little grit they do eat from their bowl - grit lasts a long time in their system, so they don't need to replenish it often. As for the oyster shell, a lot of chickens don't actually like it. They much prefer eggshell. Try giving them crushed eggshells and I bet they'll eat that gladly. They are still very young and haven't had enough time to run up a serious calcium deficiency yet. If they aren't eating enough oyster, they may be tapping into their bones' calcium reserves, which won't be noticeable for some time. If I were you, I'd offer them crushed eggshell and see if they eat that. If they do, then they do in fact need calcium that they aren't getting elsewhere, and I would consider giving them eggshells regularly. If they don't pay much attention to the eggshells, then the calcium they get from their feed or other sources is probably enough for the time being (even all flock feeds have *some* calcium in them, just not high enough to support regular egg laying). I don't know if chickens are smart enough to put aside their dislike for oyster shell for the sake of their bone health, or if they'd continue to turn their noses up at oyster shell and end up depleting their bones... Chickens aren't always bright. So I wouldn't risk their long term health. I feed mine an all flock feed as well, and they have an unlimited supply of crushed eggshell (which they love), so I know they're all set for the long run.