If I’m going somewhere in the continental US, my choices are to fly or drive. I’ll be traveling the same number of miles either way, so the relevant comparison is indeed per mile.

That doesn't explain anything. Specifically it doesn't explain why in our comparisons the destination is fixed. You could decide for this weekend trip we're going to budget a max of 3 hours on transportation. Should we take a three hour flight to a different state or should we drive for three hours to a closer destination?

This just isn’t how normal people behave. They are starting with a destination (relatives, national park, etc.) and then choosing how to get there.

That's not how normal people behave. If people choose a faraway destination first they are implicitly assuming they will fly there.

except 10,000 miles by anything but air isn't a single trip, it'd be multiple trips and involve a boat so that's not really a fair comparison either.

Furthest you can go in a straight land on land is about 7000 miles :).