In fact, on modern cars many times these panels are replaced.
If you get a big enough dent in a door, a good body shop will offer to replace the outer skin instead of filling with bondo. They cut the weld on the inside of the door all the way around, take off the shell, and epoxy a new one on. The body shop owner told me that the epoxy is actually stronger than the factory weld.
Yes, bodywork is quite a mature discipline. I was presuming the parent commenter meant user-replaceable, i.e. bolted on.
> The body shop owner told me that the epoxy is actually stronger than the factory weld.
Often this is because the special high strength steels used in vehicles today depend on proper heat treating to attain their strength, and welding can compromise this. Many OEMs even specify panel bonding for repairing particular crash-critical parts of vehicles now because of this.
It's mostly because the factory welds are the result of someone running numbers until they find the bare minimum whereas the autobody guy would rather not risk it.
The OEMs have proper repair procedures that are the correct way to fix the vehicle, and if the autobody shop is reputable, they follow them. And the stated reason OEMs specify panel bonding instead of welding is:
1. because UHSS is sensitive to heat, and robots are much more accurate in how they heat than Jimmy with a tig torch, and they were programmed by a process engineer, where as Jimmy welds until 'it looks good'.
2. welding may compromise anti-corrosive treatments on the inside of inaccessible cavities, which can lead to corrosion issues
e.g. https://rts.i-car.com/crn-24.html
A crappy shop will certainly just weld panels in without any regard for materials engineering, but it results in a crappy repair.