Poor man’s TOR :).
It’s not completely inconceivable that ISPs using CG-NAT could keep logs that would allow these users to be deanonymized, but it’s an extra step and they might not have enough information between the Reddit and ISP logs to do it. But… they’d have to be talking to the ISPs anyway, and the ISPs will probably cooperate?
This is an issue sometimes. It's not great when suddenly there's no sidewalk and you're walking alongside a busy road. That said, I don't think this is the main contributor. I think people are just in the habit of driving everywhere to the point where they won't walk 15 minutes from their house to a convenience store and will opt to drive instead, and this is in neighbourhoods with good sidewalks and no particularly busy roads. People just don't think about walking as a means to get anywhere and they'll frequently drive just a couple of blocks if they're going to visit somebody nearby too, in my experience anyway.