You buy a car, you know the price, if you drive the car for 2 years you know what the price is going to be. You usually buy a car for a reason, for example so you can drive to work etc. You have a purpose for it.
You buy a skin, valve makes an update where trading is restricted, your skin value drops by 25% without you knowing about it or able to control it.
You buy the skins because you want your gun to look different in game, you actually have no use for it.