Because they give people a lot more freedom than trains --- if you own a car. If you don't own a car but live in a society where everybody else has one you are kinda screwed.
I know it is not necessarily your opinion, but cars don't give freedom to people - even if you own one.
You pay so much for your car, the society pays a lot for the infrastructure, this infrastructure takes away the freedom from people (especially children) to live and move outside, when you drive you can (should) do nothing else, it's reputation as individual way of transport blocks expansion of public transit, ... every aspect of a car is taking freedom away from you. A car that gives people freedom is a marketing strategy and the opposite is true.
To be fair, cars are the best option in a landscape designed, subsidized, and built for cars. Most people lack the mental capacity to imagine anything different. An GPS monitoring ankle bracelet is "freedom" if your only alternative is prison.
I agree, but let's not rest on the thought that there is no alternative to the heavy car use all around the world. We got here by choice of a few and manipulation of many and it is reversible. At least if you are not living in the US, that case is hopeless.
I can go out drinking, text, work, chat, and sleep on my commute without being arrested or viciously murdering someone. Sounds like I have more freedom here.
Self driving electric cars will likely replace the role of traditional taxis. That's why they are being developed, taxis aren't a replacement for public transport and neither would fleets of autonomous vehicles. High speed rail should replace flights, public transport and bike lanes in denser population areas replace cars, and autonomous vehicles replace taxis for where public transport routes aren't available or viable.
Instead of owning an expensive car, which sits unused most of the time, you would call up a self driving car which would scoot to your location, take you where you need to go, then go do the same for someone else. We wouldn't need anywhere near as many vehicles and nowhere near as much parking infrastructure if this takes off.
The cars could all head to a depot out of town when they need recharging or maintaining. There are plenty of efficiency benefits when it comes to traffic too. The vehicles could all communicate so you wouldn't have traffic jams, and traffic lights would only need to operate for pedestrian crossings as the vehicles could flow around each other.
There's a lot of benefits to the approach. The pain points will be whilst there are still human drivers getting in the way.