Actually, they can be repaired as the aluminum sheet panels are glued together but it takes a special adhesive and skill to remove and replace them and most body shops lack the skill as well as the adhesive. Before I retired, I worked at a Freightliner dealership and our body and frame shop was eqiupped with and had the skilled body people to do it. They did it every day because late model trucks are a mix of fiberglass glue on panels and aluminum glue on panels.. They would take the damaged panel off with a heat gun, prep the joint with special cleaner and apply the 2 part epoxy, let it cure and stick on a new panel and then prep and paint the injured area. Freightliner and Western Star hoods are actually made in 3 individual pieces which makes collision repair easier and less costly. If you look at a Freightliner hood, you'll see a seam running from the cowl to the grill. That is where they come apart at. Western Stars come apart at the fender transition. The epoxy comes in a huge 2 barrel setup that requires a special gun to apply. Insurance companies pushed that because hoods are so expensive in one piece. Fords aluminum beds are totally glued together. It ain't Elmers Glue All either...lol
Because it takes skill and specialized tools, most body shops cannot do it.
What I found really interesting was frame straightening and riveted together sleeper repairs. They would strip the sleeper and drill out all the rivets with a special drill motor and rivet in a new panel but first they applied a waterproof compound to the seam. Lots of manual work involved but then crash repair on big trucks is always expensive. They would come in on a hook and leave like nothing ever happened. Same with the frames. We had hydraulic rams in the floor with retractable hooks and stantions in the reinforced concrete and they would measure the frames and pull them and make them straight again and then do alignments. I didn't work in the body /frame shop I was a diesel mechanic, worked primarily on big Cats and when it was slow I drove a truck for them. Worked there almost 30 years prior to retirement and it was a fun, well paying job and I made a lot of friends there that I see to this day.
When Cat got out of the on road diesel business, I retired.
The service manager (also retired) and I hunt together. My consuming and expensive hobby. I've hunted in just about every state as well as Canada and the house is loaded with mounts, much to my wife's dismay. She don't like the eyes looking at her all the time... lol Here are just a few Mulies I've shot in Nebraska, Montana and New Mexico... plus our place in northern Michigan
The "special" glues have been around for ages now. 3m, lord fusor, sem,sika, ect all have been making adhesives for these purposes for well over 15 years now.(possibly longer, I only started using it around 2007) From steel, aluminum, frp, to flexible plastics there's loads of options for adhesives. I haven't welded a non structural panel since I was first introduced to the 3m system. They are all very straight forward and easy to use.
The reason panels arnt typically repaired is simple cost factor, it's easier to replace rhe entire panel then patch it. That tailgate was a no Brainer for an insurance adjuster to write off as a replacment. Get the part, paint it and ship it. Least amount of time, liability, and brain power required. Same in heavy trucks, rip the old fender, cab panel etc and replace it. Takes half the skill as a good body worker, less time and the least amoint of liability. Win-win for everyone, but the guy that needs damage or rust repairs done to an older vehicle. Most shops don't want that type of work, because they know the time amd material involved and the liability if something goes wrong and it's traced back to them for a "faulty" repair.
Few years back I glued a "new" fender on my buddies L9000. (New as in it was taken off another hood with a sawzall.) took longer to dry fit and figure out how to clamp it then the actual application of the frp adhesive. Cab corners, quarter panels, dog legs, outer rocker panels, wheel arch panels. Glue them all in. Zero panel warpage, normally very little body work needs done, sand, prime, paint get some panel wax on the Inside and move on.
Some how the pictures got screwed up, the dark blue cab pics are of my 79 f350 back in 2010, doing the off frame restoration, cab corners being glued in with panel bond adhesive. Lower door skins were glued on in the same method. Didnt have pictures of the rear wheel arches, dad said the glue would never work. He welded one side in and i glued the other side in. His side took a good bit more glass to get smoothed out. I wasnt real happy with that, but it is what it is. Still looks good, and cant tell where any of the seams are. Finished it in 2012. White truck same thing cab corners. Thought I had pictures of the shops chevy 1500 pickup and c30 van I learned how to glue panels on. Couldn't find any on my phone. Most recently I did the rear quarter and dog leg on my wife's escape. My methods for clamping have changed a little over the years, but the results have always been pretty consistent.
Point being, if I can get very good results anyone with half a brain can replace panels. It's not hard at all.