There was political influence in determining routes. Congressmen were looked upon favorably if the routes passed through their districts for access by their constituents.

Tax dollars were spent, and money was made by construction companies, heavy equipment manufacturers, etc.

It made some businesses more profitable (trucking, oil, etc.) than they would otherwise be.

Could such an effort be put forth by the private sector, not the government?

Yes. What article in the Constitution allows the Government to preform such acts?

a piece of equipment

Bridges have expansion joints to allow for the expansion of the bridge

Power lines are hung loosely so that expansion and contraction can occur without snapping the lines

expect in the civilian work force, also is this job in demand for the military?

Construction workers are usually paid based on how hard they work, so it varies drastically. We don't care about your degree or what you may or may not know, we care about what you can get done.