I don't think it's that people pretended it didn't matter. They intentionally sited the pollution in neighborhoods full of Those Other People. Take, as an example, my town of Oakland, California. Are there freeways all over West Oakland? You bet. Are there freeways in Piedmont? Of course not. Rich people have historically been pretty careful to keep the freeways out of their neighborhoods. Airborne lead doesn't get very far from the freeway before it hits the ground.
Your overall point is right, though, that mid-20th-century motorist pretty much behaved like an oblivious jerk for decades.
More likely with general fear/dislike of air pollution combined with dislike of noise and high traffic itself. Living close to freeway is unpleasant, rich can afford something else, poor can not. Where it is not about about direct money, it is usually rich not poor who make decisions.
While today it is largely a question of where one chooses, and can afford, to buy or rent, the freeways haven't been here forever. They were built within the last several generations, and always located so that they would destroy the livability of lower-class neighborhoods. The freeway concept is inherently unjust; it doesn't merely reflect the injustice of society.
Your overall point is right, though, that mid-20th-century motorist pretty much behaved like an oblivious jerk for decades.