You're right. There's a lot of research and experience that suggests a well-developed rail network would have all sorts of benefits. We could take it a few big steps further and point to other problems like suburban sprawl in America, dying small towns, etc.
But there are a couple of really big, really massive problems. We have trillions in existing infrastructure and trillions more in existing property that's been developed over the course of decades. A lot of that development has come with unintended consequences. Just look at Houston after Hurricane Harvey and how their development and zoning practices took the insane rains Harvey deposited and magnified their negative consequences.[0] Decades of short-sighted decision-making took a hurricane and made it worse. Some changes will likely be made, but they won't solve nor likely keep up with a problem that's only going to get worse. That'll happen because the alternative, demolishing huge portions of the sprawling concrete city, isn't economically viable. The city can't be moved.
It's baggage. And there's so much more. Tens of trillions of dollars worth. Then there are existing roads, bridges, and other infrastructure connecting all of it. Designed for cars, and much of it legacy infrastructure that doesn't take into account modern infrastructure design principles. Add cities and towns, some of which are (charitably) in less-than-optimal positions. Towns, strip malls, and other structures that wouldn't exist if it weren't for suburban sprawl. Baggage. And then there's the people. People with ideas on what their lives ought to be like and where they want to live. There's a town, Centralia, Pennsylvania, that is literally on top of a massive underground fire that's been burning for over half a century. There was a massive relocation effort with federal and state funds. The town has no services, its zip code was discontinued in 2002, and it's still burning with all of the potential health risks that involves. There are still five residents there today, who have fought every attempt to relocate them. They will continue to live there until they die, at which point their homes will finally be taken and demolished through eminent domain.[1] If we can't bulldoze a town like that, we sure as hell can't take a bulldozer to normal ones.
Baggage. What it means is that we can't simply replace everything. It can only be done piecemeal, which means new infrastructure must take into account the need to accommodate everything that was developed under the old. High speed rail can be built out in the United States, and I genuinely hope that it is. But it literally can't replace the car in its entirety.
Self driving car research is--compared to the cost of major infrastructure replacement, let alone an alternative that simply isn't feasible--practically a rounding error. It doesn't hurt us, because the option was never a binary choice: self-driving cars OR rail. Self-driving cars give us the ability to deal within existing legacy constraints as they're slowly adapted, modified, and/or replaced. They also offer some long-term possibilities that will allow us to make radical changes in urban design. And yes, those changes are ones can accommodate rail, pedestrian-friendly city design, and other options that can drastically improve quality of life.
0. https://www.npr.org/2017/11/09/563016223/exploring-why-hurri...
1. https://finance.yahoo.com/news/pa-residents-living-above-min...