Why do you consider Uber to have the marketplace problem? If it is as simple as hiring new drivers, isn't it more of a general business scaling problem, similar to hiring more engineer to scale up more features in purely software startup?
Uber had the marketplace problem when they started but they figured out ways to punch through it, as has Lyft (our investment). Both companies have a range of clever techniques for how they did that. Drivers are not full time employees for either company so it's not as straightforward as just hiring them but money certainly helps in both cases.