It's trivial to find out why looking at driver's posts online:
There's a minimum time cost to them for each ride or delivery. There's the time you spend finding out exactly where the person is, there can be a delay between jobs, there's delays when picking up an order.
Someone who drives 10 miles for one job will spend much less time than someone who drives 1 mile for 10 jobs. And since they have X units of time to spend in the app, they'll start making less and less per mile driven.
They also don't get paid for the time spent going to a pickup unless it exceeds a certain length: https://www.theverge.com/2017/10/24/16533940/uber-long-picku...
It's easy enough to see how, at some point, they start to get paid so little that it's in their best interest to just not drive and spend the gas and their effort
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Here's a severe case showing someone go through that exact experience:
> I accepted the pick-up and app informed me that the customer was 13 miles away I drove there[...] pressed my App for the drop-off Destination [...] and saw that drop-off was 1 mile away
> Total time of trip, back and forth, 45 to 50 minutes and 29 miles , and I earned $8 dollars
> So, after thinking about it, I have decided to ‘decline’ all trips that are more than 10 miles away from me and do not say that it is gonna be a ‘long trip’
https://ride.guru/lounge/p/uber-drivers-how-far-will-you-dri...