Sure, a plane can shuttle more people from LGB/SNA/LAX to LAS. But can they also shuttle people between Anaheim, Barstow, Victorville and Las Vegas?...
Any high speed rail service is likely to have a couple stops. Even the Acela Express between WAS and NYP makes four or five stops on the way. And it's the on/off traffic which makes Acela profitable. It's not just people going from WAS to NYP, but you've also gota few going WAS-PHL, plus a few BAL-EWR, and a few PHL-NYP. There's no way the trains could be break even if they were nonstops, and the stops only account for about 60 seconds of station time plus another 60-90 seconds for acceleration/deceleration. Adds a few minutes to the trip, but also makes all the difference on P&L.