It is of course true that not everyone can afford a car, although almost everyone seems to have one in LA compared to ratios in metropolises on the East Coast, but the real reason might be something else. However, Musk's tweets reveal that car transportation will only be a priority after public transport needs are met.
The public transportation is done, according to a Boring Company video (below), by small shuttles that are swallowed by the ground and propelled on tracks to their destinations. Must says the station itself doesn't require more space than a single parking space, although a few should probably be reserved according to the video.
The main idea is to spread 1000s of little stations across the network rather than having a few huge ones like a subway system.
Better video coming soon, but it would look a bit like this: pic.twitter.com/C0iJPi8b4U
-- Elon Musk (@elonmusk) March 9, 2018