Why are big chunks of Sam Francisco missing (eg around the bridge) missing from the example?

Those smooth chunks are all (mostly) public park land. Known as Presidio and part of the Golden Gate National Recreation Area.

You know your city.

python create_map_poster.py -c "San Francisco" -C "USA" -t sunset -d 10000

-d should be approx. 10k to 14k given the size of the city. You can give it a shot.