It comes from Sera, an insane fictional world of violence where owning an assault rifle isn't already crazy enough. That might be because the average Seranian looks like a linebacker played at the wrong monitor resolution. Their belt sizes are measured in giant redwoods. It takes slightly longer to shoot one to death than to teach them the theory of relativity.
Gears of War gunfights can take longer than postal chess.
The monsters on Sera are Locusts, and their hides are so thick that bayonets get stuck in them. So Gears of War replaced the bayonet with a series of smaller blades and fitted them to a motor. The first time you try to chainsaw a Locust, the small blades will dig into its flesh and the motor will fling the Lancer out of your hands and over its shoulder, giving the attacking monster's backup a free Lancer. Your only hope is that they spend so long desperately trying to get a chainsaw kill that you can blast them with the shotgun, just like everyone else on multiplayer.