I see no reason you can't build tunnels 100-200 feet underneath people's homes to avoid kicking people out, but that has the problem that you need broad public support for spending that money. Then again, if we redirected 10% of our military spending to high speed rail, that would probably address the funding issues adequately.

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.

The reason is that it's incredibly, fantastically expensive, and in a lot of places there's water down there.