Unfortunately, what you have in LA is not available everywhere. I grew up hopping between SF and LA (lived in Novato, northern Bay Area, and Long Beach near LA) and grew up with public transportation. I could take BART and Golden Gate Transit buses anywhere I needed to go.

Unfortunately, now I live in Mississippi and there isn't even a local cab service, nevermind some kind of public transportation. Even parts of California, such as Lake County (up near Napa Valley, near-ish to Chico state), have limited to no public transportation. Lake County had charter buses and a cab service.

In both my area of Mississippi and Lake County, riding a bike / walking to and from employment opportunities, or even the grocery store, can take a good 45+ minutes, oftentimes longer for the majority of the citizens.

When I lived in Lake County, the nearest grocery store was a 30 minute drive. Now, in Mississippi, the nearest grocery store is "only" 25 minutes when driving. However, both trips are incredibly dangerous for cyclists and walkers due to lack of adequate bike lines and sidewalks.

Like I said, I grew up in some of the more populated areas of California. When I moved back [to Fremont, eastern Bay Area] for a year and some change a while back, I used public transportation to get to/from work and anywhere else I needed to go. If public transportation wouldn't work, I'd carpool or walk/bike. None of that is foreign to me. To be honest, I prefer it. Unfortunately, after moving to more rural areas of California, and even more rural areas of the United States, it became clear to me that I would need to own a car and actually have a drivers license in order to even go to the grocery store.

TLDR: Don't pass judgement on teens and young adults having a vehicle. You don't know where they live. Not all parts of the US (or even the world) have adequate public transportation or a cab service, and as such a car is an absolute necessity.

Edit: Fixed a few words.