Why do these ride companies need so much capital? They don't buy cars; the drivers aren't employees. They're a mobile app with a big backend. So why do they need more capital than, say, a game company?

You're not gona like this answer. They need capital because they are making loses year on year. Without the capital, they will be run out of money.

Marketing, software development and subsidised rides until market penetration takes off ... there must be more, but I'm not sure.