Why are train tickets so expensive?

I ride trains whenever I can, even though they can be expensive. The San Francisco Bay Area BART, for instance, charges over $20 for a round trip between Oakland and San Francisco. That's a lot of money for a daily commuter. (It's better to hitchhike instead.)

Why are train tickets so expensive, especially compared to airline tickets? The largest expense is staffing. Amtrak employs one person for every 4 passengers daily, and the average salary of an Amtrak employee is $75,000. This video by Wendover productions takes a look at this expense and the others that go into running a passenger rail company.