I was always told by people outside of LA that the reason was because in LA the freeways are so important they're revered like religious idols so they get the d

I was always told by people outside of LA that the reason was because in LA the freeways are so important they're revered like religious idols so they get the definite article. But my parents grew up through the transition you describe and back up what you said, that they all had names before numbers. I was a kid in the early 80s and back then all the traffic reports on the radio were still by freeway name, so I learned them all that way too.