Ports were and are the most effective method for transporting trade goods. It is natural that societies were founded around Port cities.

Rail is the reason interior cities can exist, but they haven't had nearly enough time for those cities to develop compared to coastal port cities.

Historically the answer is water. Today something like 90% of the global population lives within 200 miles of a major body of water. Only two hundred years ago it was closer to 95% of the global human population.

13,000 years ago when the sea levels rose 400 feet, imagine the level of total devistation.

Bikinis, beer,sun and ocean. Oh yeah,that commerce thing too.

They call them the coastal elite They say the midwest can't compete But while they flyover, I fire up the smoker, and prepare my neighbor's fresh beef.

Weather is one factor. Water has a high specific heat capacity, so it absorbs heat, reducing max temperature, and then it releases it at night, mitigating freezing temperature.

In the case of So Cal: Close access to all biomes (mnts, coast, forests, desert, etc.), Sunny 300+ days a year, and everything you'd want, commercial wise, is very close driving distance.

Jobs. Seaports mean jobs.