Without cancellation fees, drivers would have to eat the cost of whatever progress they made towards you.
While the policy varies by city, typically an UberX or UberBLACK vehicle can be cancelled for free within 2 minutes.
UberPOOL rides typically get billed cancellations immediately, because the canceling rider may have also added to the ETA of an Uber rider already in the vehicle.
Uber will occasionally waive the cancellation fee when the ETA of a POOL pickup is significantly higher than the prediction given pre-booking.
If you believe you may be eligible for a refund, you can contact Uber support. Read more here: https://help.uber.com/h/24e75a3b-cf44-44e4-abae-8c2dce3b07a3