How do you get [any rail-ish system] from point A to point B? You build the infrastructure from those two places. Uh-oh. Where? Want to arrive in the centre of point A? Whose land do you build over? How much does it cost to acquire or license? What regulations exist? What do the locals think, do they want a Hyperloop running past their window? What are the local unions like and how much will it cost to get the thing built?
Ultra-fast rail lines already exist. This isn't the reason there isn't one from downtown LA to SF (or Melbourne to Sydney or or London to Paris^ or New York to Boston).
^Interesting because the Eurorail was largely built on existing, upgraded track.