Overhead transmission line crossing over interstate highway
I posted this initially as a guess in comments, but the community seems to agree with me, so I'll make it an answer:

They designed in the ability to expand the capacity of the grid. For most of the length, it's cheaper to leave off the extra wires until they're needed. For the highway crossings, it's cheaper to wire them up now and avoid the cost of closing the highway when they want to expand.