Oyster shell grit is the most important factor in egg shell quality and strength because the shell sits in the hens gizzard and slowly dissolves during the night when they aren't actively eating and when the majority of the shell is actually produced inside the hen. Egg shells recycled back to chickens is only a quick release source of calcium, as is limestone which is often added to feed. These quick release calcium supplements do not provide calcium during the night while the hen isn't actively eating which is actually when she needs it the most.
These are the results of a study undertaken for the poultry industry to find the best source of calcium for laying hens:
"Results have been presented which show that when pulverized limestone is used as the only calcium supplement in a complete laying mash for hens, maximum production and egg shell quality are attained when the diet contains 3.5% calcium. The calcium intake of 2.8 g. of calcium per hen per day was inadequate and caused an increase in feed consumption and a significant decrease in egg shell quality. The calcium level of 5% was excessive and caused a decrease in feed consumption, decreased egg production, and yet did not improve egg shell quality above that obtained with 3.5% calcium. Further results showed that the substitution of hen-sized oyster shell for two thirds of the pulverized limestone usually present in complete laying rations resulted in an improvement in average egg shell strength which appeared to be due to a constant metering of calcium from the gizzard into the blood stream of the hens."
https://www.sciencedirect.com/science/article/pii/S0032579119538500