Do rational fears exist?

[CleanShavenJesus]

A fear of heights will keep you from getting close enough to the edge of a cliff to fall off and die. Fear of spiders and snakes will keep you from approaching one that may be venomous and lethal. Fear of the dark will make you more aware of your surroundings so you would be more likely to spot a potential predator. Fear of deep water could stop you from falling in and drowning. And so on. Fears tend to have a function.

But there are millions (if not billions) of people who don't have fears of any of those things. Why don't they, as opposed to someone else who does? Besides, your examples can easily be expressed as irrational fears, especially considering how and when people with those fears express them.

We know (well, depending on where you live) that spiders that we encounter on a daily basis are not harmful to us. Heights should not be an issue if you are safe and secure (and in many cases of those with fears of heights, it applies to any situation where they are far up, even if there is zero chance of them falling). We all fear the dark if we are in a place we've never been (though I don't see that as a fear of the dark as much as it is a fear of the place you are. The darkness is just maximizing it because you don't know your surroundings in the first place). But many people who are afraid of the dark are afraid of darkness regardless of where they are, including their own home that they know well enough to be able to walk in without a light. And what is the fear of deep water serve if we are able to swim?