So, to fix this analogy we say all vehicles on the road travel at the exact speed because a single frame on fiber should move point to point at the same speed. To make this easier we will assume that corrupt/dropped cars ("frames") cancel out and we will just not mention them again. Because the speed is now constant, this becomes a pipeline and we can completely not worry about latency.
So, now we have an autobahn whose "bandwidth" is 40000 cars per second.
The way haulage works is by the car with no real guarantee about a steady rate of delivery. I don't care, just get 10000 cars from point a to point b. This is no-deadline data transfer like downloading a file.
The way streaming works is that it needs exactly 10 cars per second with no mention of total cars. I don't care, I just need a steady 10 cars per second.
For reference, I can service 10,368,000 of those haulage requests in a 30 day month (ideally) but I can only have 4,000 simultaneous streaming users, ever.
The internet, before streaming, was almost all haulage. Most of the infrastructure was built on the idea of haulage being the dominant use case. People needed to get information from point A to point B as fast as possible. Even torrenting, was haulage. "Congestion" at peak times was not as important because delivery deadlines were looser. As long as you were averaging more cars delivered over a time period (24hrs) than requested the congestion would eventually clear.
Then streaming hit. Now most of the traffic on the internet is on a tight deadline. That infrastructure built for haulage was completely incapable of handling it. We have 40000 cars per second but this is now "Rush Hour" and we have 20k users wanting to get 200k cars in THIS EXACT SECOND otherwise their experience suffers. It doesn't matter if we unused bandwidth at 3am, it doesn't matter if we have unused bandwidth ever. What matters is that we can cover the peak cars per second requested on our network. If we cannot, we have oversold the autobahn and are now "guilty of fraud".
The Internet was like an autobahn but these days consumers only care about what happens during rush hour. Imagine how different an autobahn would be if you had to guarantee a lane for every driver that wanted it on-demand. It is an autobahn that is huge and mostly empty most of the time (your lane is your lane and if you are not driving its empty), it was also far far more expensive to build.