I understand how combat works. Having an attack roll go unopposed makes sense if you're ambushing someone or fighting zombies (because it's not like they're trying to dodge or block). But if you're trying to hit other living survivors, and they're aware, I'm pretty sure they're going to try to move out of the way or put a hand up or something. So why is melee combat against the living unopposed?

This is a very theme-heavy, rules-loose system, so my guess would be "in the interest of keeping things simple."

That said, it's a pretty easy house rule to change it if you would prefer to have combat vs intelligent enemies be opposed.

After several experiments, I think the reason it's not an "opposed check" is that it would potentially give people 2 chances to deal damage. Look at it this way:

Opposed Checks:
Player Turn: Attacks with a pistol
Player Rolls 3 positive dice, 1 negative.
Enemy Rolls 1 positive dice, 1 negative die
2 successes, 1 stress gained for player
0 successes, 5 stress gained for enemy (1 negative die, 2 player positive dice, 2 from the weapon)

Enemy turn:
Enemy rolls 1 positive die, 1 negative die
Player rolls again, 3 positive dice, 1 negative die.
0 successes, 5 stress for enemy (possibly killing them) (1 negative die from zombie, 2 player positive, 2 from the weapon)
2 successes, 1 stress for player

Without oppose checks, it's similar to this:

Player Turn:
Player Attacks with a pistol
3 positive dice, 1 negative die
Roll:
2 successes, 1 stress.
Player takes 1 physical stress
Zombie takes 4 damage. 2 from the pistol, 2 from the successes.

Enemy Turn:
Zombie attacks with it's hands
1 positive die, 1 negative die
Roll:
0 successes, 1 stress
Zombie takes 1 physical stress
Player takes 0 damage

See It draws the combat out a bit longer, and reduces the ability for an enemy to die on their turn, and makes attacking a little safer for players against stronger enemies.

Bit late to the conversation here!

The reason I believe that combat is not opposed is because combat is meant to be fast as a back and forth between PCs and monsters should be highly deadly to the PCs. I believe there is something to this effect in the rulebook but cannot find the exact quotation at the moment. The opposed tests would mean that this back and forth would almost be forced and slow down the combat.