It costs billions and take a decade. This is something to need to time very right, otherwise you can miss a shift in the market (see A380) or a new technology (like a new engine) after you started, or get a competitor to release before you. In the meantime, the current backlogs are full, so there's no rush; everyone is waiting for somebody to make a move and enjoying profits in the meantime.

But it will happen in our lifetime.