I honestly think nobody ever bothered to teach them. Driver's ed stopped being a class and started being "have your parents teach you to drive."
And even if you do a proper class, sometimes they skip a topic. I was in my 20s before I figured out how to use rearview mirrors properly to line myself up when backing into a parking spot. (How my parents did it in cars without mirrors, or with a single driver-side RVM, I have no idea.) And yeah, the spacing is definitely different for ever car.
(Which is another part of the problem..aren’t Civics now larger than old Accords?)
Somewhat. Some of the bloat is to make cars do crash testing better, and I won't begrudge them that. But small cars like the Fit, Fiesta, and Smart are still around, even if they weigh more than they did in the '70s. And they're still "subcompacts" and they're still plenty cramped. They also sell poorly in the US.
In the case of the Civic, Accord, Taurus, and a couple others, the nameplates have "grown up" with their buyers. (The Accord is probably the most extreme example - starting out as a "compact" sedan and now I'm seeing it referred to as a "full size" even though it probably technically isn't. The RLX is based on it, with a slightly longer wheelbase, and probably qualifies.)