Fake Reloading Animation

It is unknown why exactly this happens in games, perhaps it's a method of cutting corners or going easy on the modelling, or perhaps the gun models are rigidly designed and can't easily be modified, but whatever the reason, it is plainly obvious whenever it is used. When the player reloads a gun after firing it, instead of actually removing the spent clip and replacing it with a fresh one, the character makes the appropriate movements but does not actually interact with the model of the gun, miming the motions of reloading while an appropriate sound effect plays. This is most obvious with machine guns with large hanging clips; the clip is not actually removed when the character pulls their hand away. 
 
Some games attempt to disguise this, for example carefully designing the animation so that the false animation is hidden behind the character's body when viewed from behind. Others don't even make this much effort.  Some games even use a combination of fake animations and real ones; in The Saboteur, Sean Devlin drops the clip from machine guns and replaces them with a new one, but mimes the actions of reloading the various pistols in the game.
 
In some cases, it is understandable; for example, in inFamous, it is only enemies, not the player character, who use guns, so less effort would naturally have gone into the reloading animations. And in older games, made when 3D modelling was relatively primitive (the oldest game on this list is currently Martian Gothic: Unification, a PS1 game from 2000), the idea of having a "real" reloading animation would have seemed an impossible pipe dream.