Right -- the fake crab is used because the real crab is much more expensive. For a guy like me that is allergic to fish but not shellfish, that's a very important distinction, and I always ask about that if I order crab from a menu.
Tuna, on the other hand, is cheap as hell. Which is why this doesn't make much sense.
Tuna is so cheap that we used to buy it for our cat when we had one instead of cat food, which was pricey. Cat loved it just fine.
When I was a kid and a young man, I used to break out in a rash from eating shrimp. Happened all of the time.
But around age 45 or 50 until now (mid-70s) I have had no outbreaks from shrimp. Maybe the shrimp is better. Or my allergic reaction has gone away. Who knows?