Getting approval across multiple states for lines takes a very long time. The federal government and just about any state, municipality, or private land owner along the proposed route can block or delay it. The TransWest Express transmission line project started planning in 2007 but couldn't start construction until 2023, and it only needed to cross 4 states.

If the coast-to-coast railways hadn't been built in the past, I don't think the US could build them today. There are too many parties who can now block big projects altogether or force the project to spend another 18 months proving that it should be allowed to move forward.