Unless your final destination is Union Station, downtown San Diego or some other place that happens to be very proximate to the various stations in between, using the Surfliner still requires you to use another form of transport like ridesharing to get where you’re going.
Southern California is very spread out and still organized around private automobiles.
As someone who’s lived in LA for 4 years now without a car, I think it’s a trap that we’ve all fallen into to assume SoCal was fundamentally designed exclusively for the car. While, yes, automobiles have dominated the regional planning conversation from the 1960s up until very recently (LA and neighboring cities are finally taking public transit planning seriously), up until the 50s Los Angeles had one of the world’s largest streetcar and interurban train systems in the world. The city was, quite literally, built by and for transit — even its far-flung suburbs, like the town of Huntington Beach or Claremont, were built and developed as terminuses to train lines (and the train line to Claremont, on the very eastern edge of LA County, still exists in the form of the MetroLink).
There’s been some serious sprawl since then, yes, but at its heart Los Angeles is a city that is conducive — not contradictory — to public transit. And I think our planning decisions will only get more effective once we, as a region, realize that.
Union Station in LA directly connects to several subway, light rail, and commuter rail lines. There are lots of places it's still not convenient to get to, but it's way better than it was 20 years ago, and at least a few million people live within the area that's easy to connect. For example you can easily get to/from Pasadena or Hollywood.
And on the San Diego end, Santa Fe depot connects to all 3 light rail lines as well as most of the major bus routes in the city. Once the Blue Line extension opens in 2021, you'll be able to get off Amtrak at Old Town and be in the center of the UCSD campus in 15 minutes.
San Diegans tend to have a bit of a defeatist attitude toward transit, and there are plenty of suburban areas where transit is bad, but there are also many places where transit is a reasonable option.
Also, not to mention SANDAG is proposing to drastically expand the existing public transit system with commuter rail and other modes of transit along major commuting corridors, even going so far as to divert funds historically used for (with, we can say now, disappointing results) highway expansion.
Southern California is very spread out and still organized around private automobiles.