I ask why not? Dell made a fortune offering custom built computers, so why cant Moto have a shot at being the first to do this with a smartphone? I personally wouldn't mind saving maybe $100 if I downgraded the camera or something.
A smartphone is almost entirely integrated hardware while a PC is almost entirely built from off the shelf parts. It's like asking why it's so easy to get whatever toppings you want on your pizza at a restaurant but super hard to get whatever you want in your soup.