Following the highway network is great (and Germany largely does it for HSR) but the highway system is designed with very different constraints. That might be possible in largely desert or rural places without a lot of topography, but highways have curves and slopes incompatible with high speed as soon as you hit a significant topography.
The cost of right-of-way skyrockets in urban and dense environments that are exactly the ones you want to serve with HSR/HyperLoop. Both systems are on par in the regard. The mile-long tunnel to bring HSR in the heart of San Francisco is expected to cost several billion dollars. The cost of the technical equipment of that infrastructure for conventional rail is marginal compared to the cost of digging the tunnel itself.
Sure we could solve that, with strong eminent domain powers for the state for instance. But I'm not sure anyone wants that, nor that the Supreme Court would let it happen.
It might very well be that they should have spent more effort on that aspect, but they didn't ignore it.
And the PG motto is great if you want to make money as fast as possible with as little risk as possible. It's not nearly so great if your goal is to make a major change to society.