It takes time to sign up human drivers and onboard them. Buying new cars can be done in bulk at scale.
Also, you can easily send a car from one city to another. A human driver needs to get home to their family. You can't just say "drive to Miami and work there for three weeks".
Also, simply decreasing the cost per mile by cutting labor costs makes further flung areas more accessible. You can't sell a $10 ride if it costs $10 just to pay someone to drive to the area and back. But if it costs $4 to send a car, that ride becomes financially viable.