I always wonder "why can't they focus exterminating the guard before trying to pass the gate"?
Probably because they're far outnumbered, they'll probably get reinforcement called on them if they fight for too long while leaving the enemy leaders intact.
What would make the most sense is that the story is mostly written to be spectacular, and reading a long dogfight in front of the doors wouldn't really be interesting or thrilling. Plus, mc has now a special role to play, so that's like killing two birds with one stone.