Uber and Lyft monitor both the driver and passenger. The driver is more confident that the passenger will be present when the driver arrives. The passenger can be more confident that the driver will in fact arrive and not pick up someone else along the way, never telling the dispatcher that they changed their mind.
The lack of trust between taxi driver and taxi passenger ruins phone call hails. Once in Austin, I called the dispatcher for a pickup, waited for 2 hours while calling every 15 minutes and being promised a ride in 10 minutes, eventually gave up and walked 5 blocks to a major intersection and immediately hailed a cab off the street.
Since taxi passengers can't guarantee they'll get picked up, a taxi passenger will sometimes call multiple companies or pick up a random cab from the street. Since taxi drivers can't guarantee the passenger will be there upon arrival, they will pick up any random person along the way.