Theyre prepared differently and use very different ingredients. But also, sushi can be found as rolls or ngiri. Ngiri is a raw piece of seafood (usually fish) on top of rice. Dipped in soy sauce and wasabi. Sushi rolls, which is what the OP thinks this is (i think theyre called maki or something) are small rolls, like the kimbap, but have rice on the outside and usually contain raw fish. Dipped in soy sauce and/or wasabi. Sashimi is just the raw fish by itself. Also dipped in soy sauce and/or wasabi

Kimbap are rice rolls too, but the seaweed is wrapped on the outside and traditionally doesnt include meats (but sometimes contain crabmeat). It usually has various vegetables, and contains yellow radish for the flavor, so its NOT dipped in soy sauce and wasabi. Nowadays, you can find kimbap containing things that arent in the list of traditional ingredients. Tastes very different from sushi rolls.

But most importantly, kimbap is Korean, and sushi and sashimi are Japanese. The thing is, Korea and Japan are two very different cultures with different languages and different cuisines. They also have a horrible history with each other (my grandparents are korean. They were enslaved by the Japanese and my grandmother was raped. My uncle is half japanese. Some of their siblings decapitated. Everyone was forced to speak japanese. Korean was outlawed etc etc). But usually, because of Japan's more powerful influence (at least historically), most westerners just throw aspects of korean culture, for example kimbap, and call it Japanese. Idk, but its definitely something to think about. We do this for a lot of other cultures too. Like the belief that "all of Latin america eats tacos and quesadilla."