They only do that in markets where they're not established. It's just an extremely high marketing budget that gets spent on discount codes. As soon as Uber has a foothold the codes disappear and people have to pay for their rides.
Ita no different to the $10 PayPal gave people for signing up or the $7 of BTC Coinbase give to referrals really.
The fact that Uber are still bleeding money after being established in most markets they still operate in ( others were sold), would indicate otherwise.
Drivers don't pay for special vehicles (London and NYC both require special vehicles for you to use). This also means you can use a more efficient vehicle so less fuel (all the Ubers here are Hybrids), cheaper maintenance etc.
No one runs away without paying at the end of the journey.
You never get robbed because you have no cash.
Very few people will vomit etc all over the back seat or damage the vehicle costing the driver. When they do, you've covered.
Insurance is handled before you're paid, so you get a bulk discount, pay with pre-tax money and only pay for the insurance you need.
The flexibility means you only have to work when there is actual demand and it suits you to. No more sitting alone in a cab with nothing to do for hours between rush hours, you can work a 9-5 and drive for uber on the weekends etc. Earning well is a lot less when it's a top-up on your other job than when this is all you have.
2. Accountability. Uber tracks things properly.