's true that not all crabs must come to shore to lay eggs. some crabs that are fully terrestrial don't even have to come to shore cause the already live on land. these crabs are in the Sesarmidae family. They don't go to sea to realase thier larvae like other land crabs do. Gecarcinidae is a family of land crabs that does go return to the sea to breed and release thier larvae. There are some crabs that live fully underwater as well. You're probably talking about horeshoe crabs that do go on land to release thier eggs. During breeding season they migrate in large numbers to shallow costal waters where they mate and females lay thier eggs and bury them in the sand. Females can lay lay between 60,000–120,000 eggs at about a few thousand at a time. So even if seabirds do eat them they'll still be some left. It takes about two weeks for the eggs to hatch. No one takes care of the eggs after they hatch. The reason they lay their eggs in the sand is because thier eggs can't survive underwater like some other crabs. Mother's don't stop this because really what can they do? If they stay on land for that long they'll kill themselves. After young crabs hatch and they begin to grow they'll move into deeper waters.

Cuz crabs do what crabs want