Thats actually related to the idea that these stories are only recorded because they are momentous events and the storytellers wanted to get across to the listeners that idea. So when you count to 40 in base 60 you would be holding three fingers up with one hand and touching the other thumb to the middle finger. This shows to the person watching you three fingers on one hand and the middle finger which of course represents the three 'curses' which were borrowed from ancient China:
As you can see, in each of the stories the participants almost always embody each of the three. The attention of the Lord God is obvious given they're religious books about answered and unanswered prayers, but also Exodus for example the attention of the Pharoah was on the Israelites, they lived in an interesting time in that 2500 years or so of Eqyptian dynasty (scholars argue that if an exodus-like event took place, it would have been between 1000 and 500 BC, roughly during the fourth dynastic period) were interrupted as far as the followers were concerned, and finally they of course "found" freedom and Jerusalem... after quite a long hardship.
So you see now why 40 is a common number in biblical myths: it's because I've made this entire thing up.