It's not so much that sidewalks are financially unsustainable--at least if they're built at the same time as the road. But, for example in the semi-rural area where I live, literally no one would use a sidewalk if there were one. There's nowhere to walk other than to a neighbor and you don't need a sidewalk to do that. And it's going to be unusable for a few months a year unless it's maintained because of snow. (And I sure wouldn't want to be on the hook for maintaining/shoveling a sidewalk on my long frontage for something no one used.)