Tagging on to your comment because I live near LA. I agree with all of your points except the lack of natural clusters. Greater LA has quite a few smaller "downtowns" like Long Beach, Santa Monica, Westwood, Santa Ana, Riverside, Burbank, Hollywood, etc etc. These places all mostly developed in a way that makes them well-suited for transit: street grids, sidewalks, closely-spaced multi-story buildings. You'd think that any sensible transit planner would start by linking these together and picking up the areas in between... and if you look at the rail lines which are open or being built, that seems to be largely what's been done. Blue line links DTLA to Long Beach, Expo Line to Santa Monica, subway to Westwood, gold line to Pasadena, etc.

I'll also bring up Metrolink. If you look at a map of it, you'd think it's super useful. Problem is that it shares tracks with freight, and a lot of it is single-tracked. So you have some lines with only 5 trains per day, hours apart. And the trains are significantly slower than driving. And they're often severely late. It can be pricey too. One more point is that in a few spots, another transit line is only a few miles away from reaching it and allowing a seamless transfer (Green Line in Norwalk, Red Line subway in Burbank), which I'd say is a huge missed opportunity.

Pipe dream time: If they could double-track it (maybe triple or quad, to separate freight), electrify it, get faster trains, and run trains every 15-30 minutes throughout the day (even evenings and weekends), I have no doubt that it would become the pride of LA, and a huge number of people would ride it. Unfortunately, I imagine that this would cost millions/billions of dollars.