Because you would say ‘prawn scampi’?
Here, Shrimp are shrimp. If they’re big, they’re ‘jumbo shrimp’. Or ‘prawns’. Actually, shrimp and prawns are different animals
I rarely see langoustines at the supermarket, and virtually never at a restaurant. Since it’s not available, scampi made with shrimp is the default and thus ‘shrimp scampi’ is redundant. So I just call it ‘scampi’. I don’t fry the shrimps (coated in breadcrumbs, or 'fried in breadcrumbs).