Without the highways most of these areas wouldn't be able to buy or sell goods to the outside world.
(Aren't trains cheaper? Maybe we should be building more track and fewer lanes)
when in reality current transportation infra funding from federal government is about military defense.
its the same reason electricity lines, highways, and train rails don't occupy the same space. there is no reason they couldn't and would reduce costs overall. but it also means a single strike can disable all 3 systems.
in today's world I think its a pointless effort and we should resolve those issues to reduce the overall costs. but here we are.
They sometimes do, though. Namely: if you're going through the mountains, there are only so many mountain passes, and therefore only so many places where these things can get through the mountains. I-80 in the Sierra Nevada mountains is a perfect example of this; between Colfax and the 80/20 interchange (if not all the way to Truckee) they're very much occupying the same space because there's nowhere else to go (thankfully that power line's pretty minor compared to the one following 49 to the north, but the one following 49 also happens to be colocated with rail).
The more viable solution - and indeed, if you look at a map of our energy infrastructure¹, the one that we use - is to make the connections as redundant as possible. This has plenty of peacetime benefits, too (for example, if I-80 and the adjacent railway both shut down because of a blizzard or avalanche, trains and trucks can route around it, albeit with delays).
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This is generally true - typically, paved rural roads in the US are county roads. Nicely maintained gravel roads are typically county roads as well.