Probably because we don’t consume the exoskeleton of seafood, which contains chitin, in the way we do when we consume bugs. The evolution that separates modern insects from crustaceans is vast enough to make their consumption incomparable.
Why does chitin make a difference? It's tasty and not disgusting when you peel off the outside?
I mean if the concept is so disgusting, there's cricket flour and you can't taste anything different when you bake with it.
Insects are incredibly cheap to harvest, about as green as you can get when it comes to animal farming, and very high in protein.
And plenty of cultures have no problem eating insects. Oaxacan Mexicans eat crickets and they sell chapulin tacos in Los Angeles from street carts sometimes. This is a stupid Western bias that we should get the fuck over for the sake of the planet.
That's about chitin in the lungs. The body can break down chitin in the stomach. Can it cause an allergic reaction in some people? Sure. Many foods can.