If you're investing your own money, then you'd definitely want to test it out so that you can ensure that 1) it's easy to build and 2) people are willing to pay to use the tunnel.

However, if you want the government to subsidize it / provide tax breaks, then you want to be able to go to politicians in every state and show them how this tunnel is going to bring jobs and money. You pull up a big map and say, "This is going to go from San Francisco to New York, and if you are willing to help we'll make sure it goes right through your state!"

That's the political game that you play. (It also makes it politically unfeasible to shut down later. See: military contractors in every state)

"I've sold hyperloops to Brockway, Ogdenville, and North Haverbrook, and, by gum, it put them on the map!"

Or to start someplace that is much easier to develop, like in Texas. I understand the economics might not be as favorable, but there is a ton of truck traffic going N/S in TX that would be thrilled to skip over (or, I guess under) the major population centers.

Making such tunnels electric vehicle only would make them much more economical to build. Guess who is planning electric trucks?

Well in fairness I think they are doing just this. Aren't they building a tunnel at SpaceX HQ using the boring machine?

No. It would be boring.