Because it was basically 100 years ago when the car industry sought to dismantle that infrastructure, and succeeded.
That is to say, there's enough there to suggest that this wasn't the free market answering solely to consumer demand.
If you follow the same 100 year trajectory in other countries, then you will see an uptick in car ownership for sure, and infrastructure adapts to support this (highways/freeways/motorways), but it's not nearly as drastic as it is stateside.
Of course, the US is a large place and even its states are larger than other countries. And it hasn't existed in its modern form for more than a couple of centuries whereas the civil infra on the other continents has been around for at least a full millennium and most likely longer than that (e.g. Roman roads in the UK that would date back well over 1000 years). So the US got the luxury of a blank slate and, well... see what you got from that.
At the same time...car manufacturers had the perfect opportunity to seize so they could sell more cars and thus have civil infrastructure designed around the fact that everyone has a car. If someone at that point in time had more money than Henry Ford they could have dumped it into trams and trains and the landscape would have been significantly changed.
As the Wikipedia article you cited elsewhere discusses, streetcar systems outside the US went out of business at the same time as those in the US for the same reasons.
[1] No, kids, Who Framed Roger Rabbit? isn't real history
When there's funding for highways but not rails, the results are unsurprising. The benefits of public transit are a positive externality[1][2].
[1]https://en.wikipedia.org/wiki/Externality#Positive
[2]Real history is way worse
There's no public transport in the US, aside from a few exceptional places like NYC, Seattle, Boston (maybe), and, like, DC.
Uber is so popular because it actually allows you to get from A to B, for some money.
But I love the circular logic of "nobody wants the buses, why fund them" — "you can't get anywhere on a bus, why fund them" — "look how few people ride the bus" etc.
Imagine then making those vehicles larger to enable not only a decrease in traffic, but an increase in housing density, local infrastructure in walking distance, and economic growth.
Imagine if we then realized we could charge that elongated car as it's driving if we put electric lines above streets.
Oh, and why not increase rolling efficiency while we're at that, and make the task of self-driving simpler by embedding signaling infrastructure into the road itself.
You just imagined buses, trolley buses, and streetcars respectively.
Which is what makes the most sense, and will continue making the most sense for vehicles that don't sit in your garage most of the time.
There's more to car ownership than being able to get from A to B in principle.
Once you add immediate availability, individual vehicle preferences, having your stuff in the car already, the ability to keep your stuff there once you arrive at your destination, never having to think about small, but normal usage damage (especially if you have kids), or, conversely, having the vehicle just as clean as you want it...
...you will see that garages aren't going anywhere any time soon, self-driving cars or not.
And that we'd be wise to develop actual public transport infrastructure for the people who really do only care about getting from A to B, with some acceptable wait time. It's a win-win for everyone.
My pessimistic prognosis is that the development of the self-driving car will make the taxis slightly cheaper, but instead of paying your Uber driver, you're going to be paying engineers, maintenance, and cleaning staff.