As someone who took 165 through Fenton[1] from I-10 to I-49 weekly for 5 years, there's money to build a five-lane federal highway to serve logging, agriculture, and military use, or more cynically for a legislator to say "I brought $XX million in federal funds home this term". (165 is also a hurricane evacuation route, and Fenton is only 12 miles up the road from I-10, this is the typical evacuation demarcation for a major storm. That's why swaths of it are more than two lanes. Further north and between towns, it narrows to two lanes.)

In either case the liability of towns or the well-being of the people who live there do not factor into design decisions of villages whatsoever.

A footbridge doesn't serve anyone in power, and nobody locally would use it. A tunnel in south Louisiana, even that far inland, is logistically stupid even with good intentions.

1: I was born and raised in the Louisiana terminus of 165, which is only a little larger than Fenton, and Fenton is only one of about a dozen such small towns on that route.