Truck traffic Waaaay too many tractor trailers on the highways. Their size and payloads keep increasing and the trailers are so big that they badly obscure the view. More than once I missed an exit because the tall trailer in front of me was blocking the sign, and I wasn't that close to the trailer. There is nothing worse than getting stuck behind a cluster of slow trailers struggling to get up a hill.
Construction zones Waaaay too many work zones. We don't have four lane highways, we have single lane bottlenecks. They take much of the joy out of travel. Our highway infrastructure is not designed for longetivity, it is designed to perpetuate the public unions in charge of maintaining the highways. That means compromised construction. Visit other developed countries like Germany to see a stark contrast of how much better their highways are built. The last twenty years I have not planned any travel during construction season because the travel experience is too awful.
Traffic Density Trucks notwithstanding, there is too much traffic on the highways. There are enough idiot drivers that it is not safe. Drivers using their palm devices or phones, cars cutting in front of me too close, people cruising in the passing lanes, cars cruising side by side at nearly the same speed with no way to pass them. I'm not a fan of fender benders and the probability is too great on the highways.
The View What do you see from a highway? Oh look more concrete roads. More trees (or desert or flatlands). More billboards. Yawn.
If I HAVE to travel, frankly I prefer the back roads or Amtrak (I refuse to use air travel for personal vacation). The view is much nicer, the traffic is much less dense, and you see many things you will never see from a highway. It is just as slow as the highway.