Cause many people don't even realize what a "usb stick" is. It's a storage device, that uses a nand flash (where all your 1's and 0's are stored). It's not a micro controller, it can't be programmed... it can't emulate anything. It's manufactured to be cheap, and simple. The differences in them all comes down to the size and speed of the nand flashes used.
I'm beginning to believe that all these clones are coming from no more than two companies/manufacturers. This does exclude the development boards like the teensy and others. I'm referring to the true clones that's been announced. It's getting ridiculous!