In a major metro like Boston 1 minute further down the road could mean a 5 minute longer wait. I think your point stands however with 6x traffic increase to once every 10 seconds.
I'm not sure that's true. Almost every rideshare I've gotten in the last few years outside of an airport pickup has been in motion when they got assigned to me, and usually already on another ride. Even many of my airport rides were being picked up by someone who was dropping off someone else at the airport right before.
You also don't know for-sure what route they're taking. Almost every driver I've had was using Waze or some other mapping app that usually took a different reasonable route than whatever Lyft was showing when I booked it.
They're in the business of saving gas. Not cruising around idly while they wait for a ride.
Sure - some drivers are finishing a trip nearby. But you know the route they are taking...