Thing is it IS a joke. Look at the block rates and the economy. It was set up completely as a parody to virtual currencies Reddit-style from day 1. Completely tongue-in-cheek. What you're seeing is the altcoin spread speculators. It's quite literally a brand new altcoin. Maybe it takes off. Maybe it doesn't. Those that made their fortunes on the Bitcoin bubble have money to throw around. Why not pour a few bucks into every brand new altcoin when they first come out, buy when they're cheap, tuck away your wallet for a rainy day and come back 3 years later when THAT altcoin magically is worth $1400/coin (and you happen to have a few million of them in your wallet!).
The speculators aren't paying attention to the setup of the coin economy for a particular coin. They're just buying them when they see it at whatever the market is.
On the flip side, the really weird thing about dogecoin is there honestly seems to be a fledgling economy for it. It's a completely parody economy, but there is one none the less. There's already dedicated sites to exchanging dogecoin, gambling, lotto, giveaways, contests, X Prize. Hell, they're trying to make it the official currency of Reddit (or at least that particular sub). It has a freaking cult-like following. Dogecoin might actually have more staying power than most of the other altcoins out there.