I wonder if it's done this way because they need Voss and the dragon to be alive for later. If we were allowed to fight them, you can bet some people would find a way to cheese or cheat their way to victory, which could break some things.
Larian are already doing a great job at making sure that whatever the player does, the game has an answer to it. I'm sure that, if Voss dies, a solution could be found to move the quest along. The simplest would probably be to have a different NPC take Voss' place when he is dead.