It wasn’t just them, or the auto manufacture lobbyists that were probably more powerful at the time. There was also the influence of slightly older conspicuous consumption, so suburban lots were designed to look like mini country estates, and generally the re-emphasis of connection with the outdoors and nature that came in the midcentury. Plus, if it’s a totally new neighborhood, you can keep minorities out from the start.
It seems designers thought people in suburbs would like be close friends with everyone on the cul-de-sac, and they’d spend all weekend chilling outdoors and having barbecues. Maybe make one giant croquet course all down the street. Instead, you barely know your immediate neighbor’s names, and anyone two doors down is under suspicion of being a violent criminal.
To be fair, they aren’t the first or last designers to fundamentally misunderstand how the public will interact with the infrastructure; that’s still a source of surprises today.