As other replies have said, with a high user count, they've likely negotiated that per head figure WAY down. But even if they didn't, it's not as simple either

As other replies have said, with a high user count, they've likely negotiated that per head figure WAY down. But even if they didn't, it's not as simple either as "can those engineers clone this software for less than X", it's whether they can afford to take the opportunity cost of not having engineers work on something core to Uber's business.