The back story in case you care (I find it interesting):
That goes back to when California started giving names to the freeways. The San Diego Freeway, The Santa Ana Freeway, etc. In So Cal pre-smart phones, we used to be VERY dependent on the radio traffic reports (and even local morning TV) where they would go straight to the reporter in the helicopter to give them the most accurate info. That guy worked (still works) for a 3rd party that actually gives reports to several radio/TV stations and rotates around to give reports - he doesn't sit around doing nothing int he 15 minutes between reports at the station you are listening to.
LSS - the guy in the helicopter did not have time for the long-winded freeway names, and a lot of people (myself included) thought the names were silly anyway (San Diego freeway does not go all the way to San Diego - for example). SO, to shorten the report and get reports to the most stations, he just shortened it to the number. THE 405, THE 5, THE 101, etc.
BUT....
You won't find people in No Cal doing that because they hate So Cal and they don't want to be associated in any way - weird but true. So Cal steals their water, and other such nonsense. They stick to Interstate 5, Highway 99, etc just because they are stubborn.
And before you call out my regional bias or whatever, I grew up in No Cal and have lived in So Cal since the 90s. I have walked both roads - so to speak. IMHO, once you get used to the So Cal way it is the most economical way to say it and it feels weird any other way. And yes, you do get a lot of stares when you travel to Boston, New York, etc. and a lot of glares from the folks in No Cal when you go up there.