Why do people spend so much on pixels?

Little story:

So i bought about 10 keys for CS:GO a few months ago. I won a few common guns, and.. a 'special rare' item. Which was a Flip Knife Night. I never really understood the value of it. Did not even realize you could sell it. I recently got bored of CS:GO after completed all achievements, so i decided to sell up. I put this flip knife on the market for 100$ not believing anyone would actually buy it.. TEN MINUTES later, i have 100$ in steam wallet. It's laughable. I spend about 15£ originally, on boxes.. and i thought that was being stupid - buy 100$ on a single weapon.. jesus.

Not that i'm complaining, i bought the new COD and some other stuff.