Well thats where you have to balance realism with gamedesign.

Making a shooter "realistic", and looking at you character
sitting down, deassemble and clean your weapon might not
be too interesting for the game-pace.

Also reloading will take longer.
(especially since you reload a whole clip, and not single bullets. So reloading after half a clip would leave a half
empty clip then.. not a sum of remaining bullets like in all shooters.)

I would like to see how a fully realistic shooter would play
like. (including realistic reload-times, overheating,
bullet-trajectories and impact on bodyparts)
And if it is fun then at all.

(imagine, that you have to take apart and clean your MG42, and an enemy
comes up.
Then you use the barrel to smack the enemy over the head,
that would be funny)