The real problem is that the existing rail lines aren't straight enough for truly high speed rail. To build new ones, you would need broad public support (because you're going to be kicking people out of their homes with eminent domain).
The other thing is that greater superelevation would increase speeds possible through the existing curves. But as you increase superelevation, you potentially increase the minimum safe speed through that segment of track, and having 0 MPH be a safe speed is desireable. There may be some maglev derivative that could help with that problem. Rubber tires on concrete also have greater friction, which may allow them to tolerate a greater amount of unbalanced superelevation.