While we're at it, why the hell isn't there all-day, every-day direct service from Millbrae to SFO? Having a massive multi-transit-agency station right by the airport, only to tell people "nope, can't get there from here, you have to go to San Bruno and change trains" is just dumb.
Every day direct BART service between SFO and Millbrae was the original intent, and they provided that level of service when the station opened. But ridership fell well below expectations, and BART eventually limited that service to nights and weekends only.