I don't know for sure, but I have a hunch from watching our three hens go broody and hatch out chicks this spring. They all laid eggs in the same nestbox, which meant that a clutch of the right size for setting (for my small bantams, that was 8 or 9 eggs) only took 3 or 4 days. If a single hen tried to lay all those eggs herself, it would take her more than a week, and the eggs would not be as fresh when she started to incubate them, or as likely to hatch.
As soon as one went broody, the other two started laying their eggs in another nestbox. As soon as a clutch built up in that box, the second hen went broody. I moved some extra eggs to the third nest box so the third hen could start setting as soon as possible, or it would have taken the poor thing forever to lay enough eggs for a clutch all by herself.
Try leaving a golf ball or a dummy egg in each of your nestboxes and see if your hens like laying in them better.