300 miles is 300 miles, no matter what country it's in.
People don't use mass transit in the US because there is no convenient ground mass transit. US regional air carriers are the mass transit for those distances.
Do people think Japanese and Europeans don't have cars?

Wide open spaces make it cheaper to get track right-of-ways. The airlines aren't making money anyway, and alternatives (and competition) will allow them to have less scheduled flights on routes that aren't money makers from decent loading now. I think some routes are place holders for market share and gating.
I really don't know enough about the airline industry to make definite statements, but maybe someone more qualified than me would know those answers.
And noise is only an issue because the US rail system is antique. Your diesel trains on old tracks make noise, but that just isn't the case with these. These trains are electric (no motor noise) and the tracks are so smooth that you can almost play pool onboard and there isn't much exterior noise - except the wind sound as they go by if you're standing a few feet away.
There hasn't been a fatal accident on a
shinkansen in 40 years of operation. Not a bad safety record.
Darn lazs, why would you live in a country that was as bad as you make it out to be? Slums, stolen cars, bureaucratic nightmares...