This is something that bugs the shit out of me. In everything, because it happens in everything.
To boil this idea down to the basics, it's plot convenience. Why don't the Covenant just glass every single planet with humans on it? Why don't (good) wizards just use the (immobilization) killing spell and be done with it? Why doesn't Jon from Garfield just kill himself? Well... because.
That'd be boring.
I tried to find the specific TVTropes article for it, but it's impossible to find the one thing you are looking for on that site, so... well, nevermind that point, I guess.
There was a scene in Castlevania: Lords of Shadow (don't worry, not a spoiler), where you are fighting crazy wolf things while racing through a forest. During this section, you are riding on top of a... horse (I don't remember), and attacking these wolves, traveling at like 70 miles an hour. To kill one, you have to JUMP OFF OF YOUR HORSE AT 70 MILES AN HOUR, on to one of them. Then you have to kill it, causing it to --obviously-- fall over dead. Then, within a split second, you jump off of a no longer moving carcass, back on to your speeding stallion, and keep riding.
All with out missing a beat, all with scary instantaneous reflexes.
But then.
The section ends, you kill all the wolves, and a little cinematic starts playing. You, on your horse, running down the very wide forest path. You look around, to see what's up with the wolves. They say "not much", you say "okay, cool", and you face ahead. You gasp, see a tree branch straight in your path (at just the right height to hit your body and not your horses, causing you to be knocked off of it). You see it coming.
You see it coming, and then you see it coming.
And with lightning quick reaction time,
You get smashed in the face.