Why do generally lower level items are worth more?

Basically every level 20 and up legendary item sells for like 200gold

I dont have a problem with it; nets me a lot of gold but after going through many different games with their own economies (like tf2 for example, or dota although ive never played it), this one confuses me a whole lot and im hoping an experienced trader, or auctioneer in this case can explain it to me