We have had different experiences. We are Executive Platinum member with American Airlines, and the FA at AA are always able to look up an ingredient list for the food they provide. We have also had similar experiences with British Airways. So you are wrong in your belief that no airline in the world would be able to provide such a list. The FA may not know it from the top of their head, but it should be possible for them to look this up if necessary.

On your second comment about how people with allergies should always make sure to have their own food: It's impossible to always be 100% prepared 100% of the time. That's why people suffering with allergies need to know that they can rely on alternatives when possible. You wouldn't tell people in wheelchairs that they should always carry a lift around with them to get into places, right? Suffering under allergies is a disability like many others, and not expecting that airlines are prepared to deal with them is part of the problem. Having an ingredient list is not a big expectation. They have such lists at fast food restaurants that get all their food catered - why wouldn't an airline have it?