I forgot to put in my comment originally that I was thinknig of fences (I just added a fence which prompted me to make the comparison)

Siding would require a different style of mounting, but it's certainly not impossible. Nobody is really looking to do this at the moment, but give it 5-10 years and we may see more. Solar roofs have been mostly a boondoggle so far, but nobody has seriously tried them. Tesla/Musk don't count for "serious" on that axis of development.