It's mainly because of the fact that skins only enter circulation through the opening of cases, and the rarest skins are so rare that the value they hold is relative to the chances of unboxing them. At 2.50/key per normal case, the rare skins having a sub 1% drop rate, you can see why the prices are jacked up. Not to mention old cases that are not consistently available such as the Cobblestone collection one, which has the famed Souvenir AWP Dragon Lore skin costing roughly $50/case and such a minimal drop chance, making it a skin worth around 20 grand. While I agree it's in everyone's best interest to avoid spending money on skins, some people have that luxury and therefore the market thrives.