BART is already automated. The "drivers" just make announcements, check that the doors don't close on someone's leg, and press the emergency stop button if the computer goes badly awry.
This is the same kind of "automation" that is in place at most mass transit systems in the US (sans DC, which still uses human operators). The requirement of having that human operator is typically born strictly of labor requirements first, and not necessarily the need of having someone's hand over the "Oh shit" button.
Sans New York also -- with one exception, all of the MTA's trains have human operators, and the majority have two-man crews. It has been possible for a single person to operate the MTA's subway trains for decades, but attempts to do so on "full length" trains (most lines; shuttles and the G are the main exceptions) have generally resulted in failures to meet safety requirements e.g. evacuating passengers quickly enough during a fire. It is overly simplistic to claim that this is strictly about labor requirements; while the TWU certainly works to keep its members employed, there are many other factors, at least in NYC. Also notable are the various attempts to increase automation, which have failed not because of the union, nor because of technology not being up to the task, but because the MTA does not know how to hire competent engineers for its projects (despite having billions of dollars available to pay them).