Well...I guess you ask that question about anything...kwim? Why is anything so expensive when the knock off is cheaper? There are just plenty of people out there, happy to pay full price I guess! I don't actually think they're that expensive...but we're all different in that aspect!
They do go on sale at department stores (saks.com, neimanmarcus.com, nordstrom.com etc, dillards.com, lordandtaylor.com, macys.com, bloomingdales.com etc) and you can find decent deals on them at shoe sites like Zappos.com, Shoes.com etc. They sell them all over the place online, so you just have to keep your eyes peeled. Are you going to find a pair for $20? No, probably not....so perhaps just get the knock off?
I bought a pair for our babysitter for her birthday a few years back. She was SO excited. LOL

I am pretty sure I got them at smartbargains.com or overstock.com but that was a few years back.
They do get dirty VERY easily...esp the lighter colors.
If you have smaller feet, consider buying the largest children's size.