You need delicate sensors to detect where every vehicle is, to check for fallen branches, that confirm that your switch points actually moved, to communicate with each vehicle, and a thousand other things. All of this needs to work in heavy wind, rain, snow and ice. All of your delicate electronics need to handle massive shifts in temperature, and the resulting expantion and contraction is indeed the primary cause of failed electronics. All of this is exposed to all of the above day in day out for decades.

You can do it, indeed many automated metros and such do exist above ground, but it does significantly add to the cost of everything while dramatically reducing lifespan.

You can’t just put it in the middle of the road becuse you need to support it, and if supports are near cars than they need to be very big cast concrete structures in order to survive the inevitable crashes. You can build big supports and massive beams across the roads to support the track, but all of this adds about as much cost as just trenching it in the first place. There is a reason we don’t put our water mains above our streets.

People do very much care about what gets put above their skyline, and such a system would only really be practical if you were to expand it to residential and commercial areas, after all the primary point of the system is delivery speed, and nearly all industrial goods don’t need same month delivery, much less same hour.

Finally, the ground already often looks like swiss cheese in many cities, it’s just earth is good at spreading out loads and it’s very easy to carry the weight of that earth and the stuff above it with a few inches of concrete. Again, we run large water and sewer lines all over the place.

While this system is unlikely to be practical, Chicago had a similar idea over a hundred years ago after all and abandoned it in favor of trucks for a reason, putting it on stilts over everyone’s heads is not liable to make it more practical.