One other aspect here in SoCal... There are a lot of major freeways that are NOT part of the interstate system, they're California highways (or 101 which is a US highway not an interstate).
So in the LA Basin you've got I-5, I-405, I-10, I-210, I-710, I-605, I-15, I-215, I-105, I-110 etc.
But you also have major freeways of CA-91, CA-60, CA-22, CA-55, CA-2, CA-57, CA-134, CA-118, US-101. And you even have CA-110, which is the northern end of I-110 but isn't officially an interstate designation.
That's a LOT of major roads. Moreso IMHO than probably any other metro area in the entire US.
It's unwieldy to name them all by the I-## or CA-## designations. Whereas growing up in the Chicago area you might tell someone to take I-88 to the Eisenhower to get into the city from the western suburbs, here you might literally have a route to avoid traffic like I-5N to CA-22W to I-405N to I-605N to CA-91W to I-710N to I-105W. That's a pretty convoluted (but routing around heavy traffic, plausible) route for me to get from home to LAX.
It'd be a mouthful to tell someone to do it that way, and then you have to remember which are interstates and which are California highways. Using "the" for all of them is just easier.
