Right. You go to the opera, you order the car to drive around the block for two hours and then pick you up. Cost of gas is cheaper than parking and by definition, you're not affected by any traffic congestion caused as you're sitting watching your show. Sure, 1000 other people in the theater are also doing it, but that's not your problem.

30 minutes before the show is over, you order the car to wait in the "no stopping" zone in front of the theater for easy access. You exit the theater and find your car is tangled in a mess of traffic that takes 45 minutes to clear as most of the cars are double-parked and blocking traffic, waiting for their owners. For the duration of the show, no useful traffic has been able to move for a mile in any direction, as empty cars are just idling around and are content to wait, fully stopped.

Tragedy of the commons.

The “circling the block” is something that Uber etc. already exacerbate.

I think one solution would be to charge for each minute you are in a congested area to avoid having a whole bunch of empty cars clogging the streets.