The reason is mainly humidity. Most home incubators are quite small, and when you open it the humidity drops drastically and then can take several hours to climb back up. The chicks have to do a lot of wiggling about in order to hatch, and when they unzip they need to be able to rotate inside the shell to keep unzipping. If the whole egg loses too much moisture in the final days, the inner membrane sticks to the chick like they've been shrink wrapped. It dries to a tough leathery membrane that traps the chick in the shell and they won't make it out without assistance. By keeping the incubator closed the last few days, all all through the hatch, you prevent any chick from drying out so much they can't hatch. Back in my spring hatches, I had to open the incubator each day during lockdown to add more water because it was SO dry in the house the reservoirs were empty in 24 hours. So I violated the lockdown rule, but I went as fast as I could and opened it only when absolutely necessary. It worked fine, I had great hatches. So the lockdown rule CAN be broken, but you'd better have a darn good reason, haha.
Thanks for the great answer.
My last egg had a pip and no activity. We just helped it open so it may be from us opening to take out the other chicks & putting in water. Don't know how quickly they dry out. I hope this poor little guy makes it. The membrane was stuck and kind of tough---it was very scary trying to gently pull it away. We are watching the baby in the bator now. I want to take it out to the broody but it's so tired and weak.
More patience needed. . . .