That's not the point. The point is what they are producing isn't contributing value to our lives, or actually creating negative value. Building highways looked very productive on paper in the 50's, but now we live in car dependent sprawl, and we continue to pay people to maintain that sprawl when we could have walkable cities that are human friendly.
I love it in the city, can't imagine living where you literally cannot walk anywhere - but most of those people out there like it or they wouldn't be out there.
So, no, the people maintaining those highways aren't doing nothing. Maybe they're doing nothing for you. Maybe you can argue you're subsidizing other people's lives. But you can't say that people maintaining highways are doing nothing or are net negative.
Our preferences, choices and decisions have ripple effects.
Maybe if we'd intentionally designed our society/environment differently, we wouldn't need so many highways, and so many people managing them.