Honestly, it’s because people can be allergic/sensitive/sensitized to pretty much anything, so such a warning would basically end up being “Warning: Contains ingredients it is possible to react to.”
I’ve heard from people with allergies/sensitivities to almost every single ingredient used in cosmetics. Everything from food allergies that preclude certain carrier oils to alcohol (fatty or otherwise) to shea butter to aloe to vitamins to synthetic fragrances to certain essential oils. Allergies and sensitivities are a highly personalized thing.
If you have an allergy or sensitivity, it is up to you to make sure you’re not using ingredients that will trigger a reaction. I always clearly list the ingredients I use (that’s a must for a recipe!) so you can do your own research. It should be impossible to hide ingredients in something you make for yourself 😉 Just make sure you are checking INCI listings as well (basically, the ingredient list for your ingredients) as some products we use have trade names that are much more memory/pronunciation friendly than the full INCI.
Posted in: Safety