Some games actually give some in-world explanations for these (such as rapid rotting), while others just crudely remove the corpses from the gameworld.

This is usually done to save system resources by removing the corpses from requiring processing power still - although this is usually a significant lacking in the game engine itself as corpses shouldn't take much any more processing power than regular level ornaments - but this often also makes less immersive experience for the player, to which experienced players likely have become jaded to.

Examples:
* Winks out of existence - done in older games
* Sinks into the ground until they're no longer visible
* Dissolves or "beams out" - usually with pretty bright lights
* Explodes leaving nothing behind
* Corpse disappears after a while when you aren't looking or nearby