The skin you posted is an example of why it doesn't happen.
It's just an edit and not a great one.

If I see a gun that is better than something I think I could do then i'll add it. Even the concept, prime example is the FMG9. Ghost/#5 did the rough shape of the gun with the flip case sprites. The gun was completely black and had no detailing but the shape was good and so were the case sprites so I added it shading and details and added it to the game and to their accounts. Guns are something I want to remain having a consistently high quality and consistent quality is something you will not get from player skins.

I'm not even trying to sound up myself when I say that, just think if you are someone incharge of a specific thing, why would you accept and upload a skin that is of a lesser quality than you yourself are capable of? Sure the person that made the skin might be happy, but overall the people within the server that buy the gun are subject to something less than what would have been put out if it was done myself.
That's why there are some guns in the game I didn't make that I haven't changed/ updated as I don't think I could improve anything on them.