Why do adults lose their fear of the dark?

Many adults do still fear the dark, they don't fear it at home like a child might be afraid of a dark bedroom.. they fear it when they are out late at night, because even though we got rid of natural predators like wolves and bears, there are 'human predators' aka criminal scum that are more likely to get up to no good under cover of darkness. 

I wouldn't say I lost the "fear", but I don't feel unsafe against the threats that might happen, in most situations.  The fear is mostly of the unknown, and for children, almost everything is still unknown. It does take a while for the sense of permanence to take hold in the developing mind, and for experience to show that unexpected hazards are very unlikely in most situations. The place is not any different just because there is no light.

I ain't the dark that is scary.  It is the unknown danger which might be in the dark that is scary, whether that is an unseen hole or an animal, or more likely than anything, another human with bad intentions.

Being afraid of the dark often starts in childhood and is viewed as a normal part of development. Studies focused on this phobia have shown that humans often fear the dark for its lack of any visual stimuli. In other words, people may fear night and darkness because they cannot see what's around them.

Adults know more about the world than children. They know what to fear and what not to fear. It is natural to fear the unknown.  That is why many adults are scared of the covid vaccine.  They do not know how it is made and some of them think they may get the virus infection if they take the vaccine. Sometimes children endanger themselves because they have no idea what is or is not dangerous. Lack of knowledge is the reason why we sometimes fear something we should not, and also why we often do something that is dangerous to us, such as not wearing a face mask or social distance during the pandemic. IOW, fear of the dark, fear of vaccines, and not wearing face masks are all caused by lack of knowledge. 

Because the fear of the dark does not always concern darkness itself; it can also be a fear of possible or imagined dangers concealed by darkness.

They don't lose fear of the dark, they lose their unreasonable fear of the dark. Kids can be afraid in their own beds fearing monsters and such. Adults understand that there is no need to fear nonexistent monsters and predators.

We would have used moonlit nights to go out hunting.

They do not lose it, they just have more experience to deal with it

Fear of the dark isn't about predatory cats, it's about the unknown. As you age you figure out that nothing is hiding in the dark places, it's just dark.

Many adults are still afraid of the dark.