The FDA has special rules exempting some ingredients from special allergen labeling when the item is highly refined. An allergic reaction is generally caused by the body reacting to a protein in a substance. These proteins can be in very tiny concentrations and still affect a person.
One chemical process that has been done for centuries has been distillation. A mixture of ingredients is heated until it boils. Some of the liquids in the mixture will evaporate at various temperature. These vapors can be condensed like dew in cooler parts of the still and collected. Solids including proteins are generally left behind.
Because of this processing very few if any allergens can be found in the condensed vapor. Distilled liquor* does not have to be labeled as containing corn even if it was made almost entirely of corn and water. Highly refined soybean oil also does not have to be labeled because most people who have allergies to soybeans should not react to soybean oil. I frequently see items where soybean oil is listed in the ingredients but is not listed in a “contains” statement.
* people with food allergies should be cautious however because most alcohol containing drinks are exempt from ingredient labels. Most distilled liquors have ingredients added back into the process after the distillation is completed.
Also foods like wine and beer have large exemptions from the labeling regulations. Because I am allergic to wheat I would like to be able to read the labels of beers to see if they contain wheat. Almost all beer has wheat but almost no beer has ingredient labels.