I thought between Google, VLS, Tom Tom, and I can't imagine that I just coughed up a complete list, there'd be enough competition to just procure this function competitively. I assumed it was more the Not Invented Here syndrome of a VC funded company who (until recently) had little of a cap on their Engineering spend.
Most drivers use a 3rd party app in practice but uber probably needs to run mapping to avoid a source of weakness/cost.
(Could be totally wrong about incentive structure)