Good point, cats actually grow bacteria to kill birds with.
My mom has celiac, which means her immune system attacks her if she eats even the slightest trace of gluten. If I had fish food with gluten, I'd have to wash my hands very thoroughly after feeding the fish, plus the tank water would probably be contaminated. Which would mean that our bathroom sink would have gluten on it (Python water changer), and so would several surfaces in the room.
And when you have celiac and don't eat gluten for awhile, your immune system goes absolutely mad if it detects any gluten at all, mistaking it for a returning infection that needs to be nuked. Considering that it would make my mom quite sick for several days if she unknowingly touched something that had gluten on it and then ate at any point soon after that, it's not worth the trouble. I have gluten-free food for most of my other fish, and since I can keep my bigger fish fed and happy on frozen food, I'd rather not risk it.