Can the phone be used with prepaid plans from Verizon? Or with pay go plans from AT&T?

I am interested in an iPhone 5C but prefer to use a prepaid service from either Verizon or ATT. Since buying the phone means I own it outright, why don't the buying options include selecting a prepaid or no-contract plan? It just makes sense that now that the iPhone is available to a new demographic with less money for the usual iPhone, it only makes sense to make the phone available for pre-paid plans. Thank you.