I was an extremely picky eater as a kid. The funny thing is how I became less picky after I went vegan (in my teens.) It tracks with how we lose taste sensitivity as we age - children have more taste buds than adults. It serves them because a lot of things that can be harmful are bitter (poisons are often bitter) or sour (spoiled food goes sour), and for a small child a small amount can be more dangerous than for an adult.

Anyway, as others have said, it’s a combination of texture, smell, and taste. Being a “super taster” isn’t as fun as it sounds - it means a smaller amount of flavor can seem stronger for you, and can easily become overwhelming. I still prefer bland foods sometimes for that reason. Light seasoning can go a long way. Texture can be awful or amazing, sometimes I crave a food not for its taste, but for its texture. As I noted in another comment, mushrooms have a texture that repels me. Eggplant also has a texture I struggle to tolerate, but I put up with it when somebody prepares something for me with it (occasionally people prepare vegan foods to help me feel included, but they don’t usually know my texture issues and I don’t want to be rude.) Hell, half of what drove me to veganism was texture, smell, and taste-based - a lot of animal products have textures, smells, and tastes that I couldn’t tolerate when I was a kid and made to eat them.

Eggs smell awful, and the fat from meats have a texture that was too much for me. I remember sitting at the dinner table and cutting all the “fat” from my meats just to isolate a single square to eat… but my parents made me sit there and told me to eat the whole thing, even if we sat at the dinner table for hours. I never liked bacon, and I know the internet hates me for that (well, they already hate me for being vegan, but still), since it’s pretty much all fat even if it’s crispy. The smell does it no help, either. A lot of people enjoy the smell of cooking meat, but I never did. The closest thing would be chicken, but ever since I had pet birds, it became awful, too - ever smell a bird up close? They smell like warm chicken. Once that smell became associated with my pets, whom I lost too soon (they were only two years old, victims to teflon poisoning) I only think of them. Giving up meat was part of my closure after they died - it brought me a peace that I needed, and has continued giving it ever since.

Please be kind to so-called “picky eaters.” Their sensory experiences are outside of their control. Don’t do like my parents did and try to force it, as that helps nobody. If you want to introduce a picky kid to a new food, be gentle and start slow. That’s how we do it at my job (working with autistic children, some which have extremely restricted diets that don’t provide proper nutrition.) It starts with tolerating it being on their plate, then progresses over time to them touching it, then to putting it to their lips, and only after all that do we encourage them to try a piece. It can take months. But if it’s something they absolutely won’t tolerate near them, just try something else - there can be very valid reasons they don’t want a particular food, but thankfully there are a lot of options out there.