All people want to see the applications and normal work, so to see how it can handle for example multitasking or what applications would they have if they want to buy it.
If someone want to make android with simuliar UI they can do - as there is emulator with SDK as well as at some point they need to sell phone and UI would be revealed. It would be soon enough while it would be fair for future customers to see what they get and not paying money blindly