A major issue with relying on drivers is that if the real-time position of a driver is known, you can see both what they're doing and what they're not doing. Which is bad if you want to get away with, say, taking longer breaks than you're supposed to.

Anecdotally, at a local system I once used there was an issue where drivers were intentionally not turning on the locating tracking, or configuring it incorrectly, because of this.