Airlines are mass market common carriers that are designed to cater to millions of people. They have special meals for pre-order that accommodate the majority of dietary restrictions, including vegetarian, vegan, kosher, dairy free, diabetic, gluten intolerant and more. Between cross contamination and the inability to provide for n! / (r!* (n-r)!) combinations of allergens, there is only so much that can be done.

If you have 5 allergens collectively that are such a severe issue that someone cannot work around it, bring food from home.