I assume by "it's cheaper", you mean that an Uber competitor would charge their customers $X, pay out $Y (> $X) to their drivers, and subsidize $Y - $X by pouring in cash from their other products.
This sort of cross-subsidy has a poor track record when used against well-capitalized opponents. (In many situations, it's also illegal: see "dumping".) Unless you can complete wipe out the competition and force them out of business, you're pouring money down the drain, and building nothing of value with it. Bing Cashback resulted in most people doing their searching on Google, identifying what they wanted to buy, and then buying it on Bing so that Microsoft would pay them.
You get very odd arbitrage situations that basically result in funneling money straight from the corporation that's being idiotic to a savvy consumer. If a company did what you suggest, I would immediately sign up with them, along with my fiancee. We would then use the app every time we took a trip together or picked each other up. Since one of us is the driver and one is the passenger and the driver earns more than the passenger pays, we'd be making money at this company's expense every time we drove somewhere. Now imagine every carpool, group of friends, or just random strangers who setup a business to exploit this arbitrage opportunity doing that.