rPI is quite cost optimized. That’s one of the stated goals. An extra component to buy and place on the board costs read money. Plus, an implementation of UEFI will not be tiny. It would likely would need a few MBytes of flash. That's a dollar or two already. If everybody’s requests, each of which would add a dollar or two to the board, were fulfilled, you’d be looking at a much more expensive board, and complaining about cost instead. :)