The same argument can be made for engines, if you don't build enough cars per year there is not point in designing and building your own but if you do it becomes a competitive edge or at least a differentiating factor. For example some brand's cell chemistry might be more specialized in cycle life while another in energy density, the first might charge at q more constant power the other might peak and taper agresively, etc. At any rate, once making your own becomes the only way to scale production, one will need to invest much more money to be competitive in another decade compared to today so the longer the wait the less time to catch up in volume and expertise. It's not impossible it just costs more and so will be cars ultimately.