I ran around barefoot all through childhood and I even accidentally stuck my head in a hornet's nest. I've had unnumbered stings on the bottoms of my feet, and eleven stings in my scalp. I have no phobias of bees. I think it is because I have a realistic and healthy respect for what might happen, and I know it is all survivable.
I read that Koko the gorilla developed a fear of alligators, though she'd only seen one in a story book. Those snappy jaws just...looked...scary.
But that's it isn't it? We're not all the same. Yes humans have traits in common, but the level of fear in one person regarding one subject is not the same as it is in another about the exact same thing.
Okay, as you are with running around barefoot, I am the same way. I've been stung, bit, scraped, bled (104 stitches on my forehead) and god only knows what else. I don't care. Yet my son, who has never had anything of the sort happen to his feet, will not walk barefoot through grass. To him, without shoes, it just looks scary.
I, on the other hand, cannot sleep with my bedroom door open. My son can. Of course I had some rather intense experiences involving bedroom invasion that he (thank god) has not experienced. I think it's fair to say I have a phobia about sleeping with an open door that lies in an extremely intense experience. But my son has a fear of walking barefoot in grass based on appearance/expectation/hearing stories of bad experiences, etc.
For him, that's enough.
So I don't know if it's fair to say phobias are unreasonable. It may be that we, on the outside, cannot see or hear or feel, what the person on the inside sees or hears or feels (or remembers). As you imply, the connections in the brain pathways, why two can experience the same thing and yet remember and feel it differently is not easily explainable.
I submit a possibility lies in what you said about Koko the gorillas. The level of fear is greater in one person toward alligators (or grass) than it is in another. Not necessarily because of a bad experience, sometimes things look bad and we can make a mountain out of a molehill inside. Or maybe it touches on another issue, perhaps reminds us in some way of some other trauma.
Or not.
Chris