Probably because most cities are so spaced out you'd still need a car to get from home to the train station and from the train station to your office when taking the train to work, for example. So it's easier to just drive there.
The US population is fairly concentrated around the 'edges'. About 40% of the population lives in a coastal county:
* https://ecowatch.noaa.gov/thematic/coastal-population
And two-thirds of the population with-in 100 miles (160 km) of the border:
It's not like 1905 when you could just walk from your home in Smalltown USA to the local train station, buy a ticket, and get a ride to the nearest city, and get out and walk from that station to interesting places.
Any train trip, even if you look only at the eastern states, is likely to require a car ride on one or both ends to get to/from your source/destination to/from the station. If you have to drive an hour just to get to a train station, and another hour to get from the destination station to your final station, it's probably faster and easier and much cheaper to just drive the whole way. Don't forget dealing with parking, car rental, etc.; you'd probably have to take taxis, and those are quite expensive.
The fundamental problem here is density. America doesn't have it any more.
(It's a figurative example I'm not sure there's a train from Seattle to SF)
I'm just saying, this is such a rare use case that it's not as high of a priority as expanding the roads that 80% or more of the residents in a city use daily. Whereas for freight it makes a ton of sense.
(fun fact, there actually is a train route there!)
As some who used to travel for meetings quite a lot to a city 3 hours away by high speed rail, it really isn't. Once you take into account that you can show up for your train 5 mins before it leaves, plus the fact that the train station is almost always much closer to where you want to be, the difference in time between trains and planes pretty much disappears for shorter trips.
Plus the train is just so much nicer and more comfortable. It's quieter. Your seats are much bigger and have more legroom than even the nicest business class seats. You can get up and walk around if you want. You often have a restaurant car where you can sit and grab a drink or something to eat. Train travel is just so much more relaxing compared to flying.
Even for flights which take 45 minutes in the air, I’d never expect to get to the airport, through security, through all the boarding and unboarding nonsense, and from the destination airport to where I was actually going, in 3 hours.
IIRC last time I was in Seattle airport, after I got off the plane (which was late, of course), I spent half an hour just walking through airport and to the rather inconveniently located light rail. Everything involving flying takes forever.
You just can't do that in the US, outside some very select situations (like going from somewhere inside DC to Manhattan NYC). From SF to Seattle, how do you get to the station in SF? In Seattle, how do you get from the station to your destination? What do you do to get around in Seattle? Generally, you need a car, which means renting a car, which is really expensive. The US is set up to handle this at airports pretty well: you get off your plane and go to the Hertz counter and pick up a car (and then after your trip is over and you've returned the car, get arrested for auto theft when Hertz reports your car as stolen--don't use Hertz). I haven't tried trains on the west coast, but on the east coast, I've never seen train stations set up with rental car counters.