The real cultural difference is that, in Manhattan, all but the very richest tend to take the subway. In Los Angeles, all but the very poorest tend to drive. The availability of rail transport is not going to make ONT or Palmdale popular IMHO.
As far as the locals go, that might be true. However at an airport you also got to keep in mind the non-locals, the people for who your city is the destination. Los Angeles sees a significant number of visitors from elsewhere, both tourists and business travelers.
As a matter of fact, a few years ago I visited Los Angeles myself. I live in the Netherlands, all the way in Europe. Los Angeles is not local for me, I had never been there before and I knew nobody there. As a tourist I chose to travel by public transport which appeared to be very convenient in Los Angeles, I still have my tap card. For my stay I had a week unlimited travel pass for $25, this turned out to be the best option.
For me, it wouldn't have made any difference if I would have flown into LAX or Ontario. However the airline happened to fly to LAX, so that's where I went. In a way this was inconvenient because LAX is a way too large airport. As a foreigner with no special status except for ESTA, I spent about 2 hours clearing immigration at TBIT. I guess it's very convenient if you have global entry, however it sucks if you don't. I got the feeling clearing immigration at Ontario would have gone much faster. Then I had to get on the FlyAway bus to get to Union Station which also took about an hour, not really convenient and it added to the cost of the trip. At Union Station I bought the tap card which I used for the remainder of my stay in Los Angeles.
Should my airline have flown into Ontario instead of LAX, I would have cleared immigration much faster. Then I would have taken the shuttle bus to East Ontario Metrolink station and from there on the train to Union Station. In the end, it might even have saved me some time. For the way back I would have taken the same route, only in reverse. Now instead of taking the FlyAway bus, which is not included in the unlimited travel pass from Metro, I decided to take the long route. First the gold line from Heritage Square (near where I stayed) to Union Station, then the red line to 7th Street/Metro Center. From there on the blue line to Willowbrook/Rosa Parks and finally the green line to Aviation/LAX. And last but not least the shuttle bus to the airport. Unlike the FlyAway bus, which would have cost me extra money, this was all included in my unlimited travel pass, but that many changes just to get to the airport is not exactly desirable. I could have gotten to Ontario Airport with fewer changes, however my flight happened to go from LAX. It wasn't my choice, it was the airline's choice and I just had to comply with it.
Renting a car has never been an option, I didn't drive at all during my stay in Los Angeles. Why would I? I guess it's different for locals but like I said, on average about half the passengers at an airport aren't local.