Why are foreign food more expensive than local foods?

I'm a local, and I like eating foreign food such as hamburgers, pizza, spaghetti, stakes and i want to try each foreign food at least once, but something i noticed is that stuff like hamburgers, pizza, sushi and stakes are very expensive compared to Vietnamese food, so as much as i like them, i only eat them on special occasion.

Why are they so expensive? The ingredients used are the same as the Vietnamese foods right? For example sushi is just a bit of fish on top of rice, Is it just because it's exotic so they charge more? Or is there something i'm not seeing?