One of the other things is that a lot of the food delivery people don't actually live in SF, so they drive from out of town and do Uber things and then go home. So that also makes it harder to support more sensible logistics.
This is the real reason for them using cars. In NYC, those delivery guys on e-bikes and scooters live within a reasonable distance (via bike or public transit) of where they’re delivering.
The picture of that car-snarl leading that article implies it's a lot of cars (lots of scooters and bikes in the UK too - I doubt the cost of fuel and keeping a roadworthy car in general, makes cars economical for it here, not to mention the need for a licence which you don't need on most small bikes).
I don't get delivery super often, but for SF, my estimate would be 80% car, 19% moped scooter, 1% bicycle.