The problem is not the manufacturers that don't want damage, that's mainly a Gran Turismo philosophy (which to me makes no sense, as they could simply add an option and who likes the cars to be indestructible disables damage). The main problem is the amount of calculations required to make a good damage model. You don't want to simulate sponges like in BeamNg, which reminds me a lot of Algodoo or Phun, similar programs to play with physics. You need to take in account the resistance of the materials, the defibration of the chassis and the deformation during the impact. All of this can require a lot of CPU power. Also from the point of view of the graphic engine you'd probably need to implement bones for your car meshes to let them interact with physics. This may actually be possible to some extent. However a complete damage model includes energy absorbing zones made by the manufacturers with crash test results, so patented data, authorizations and so on are required. If back in the day Kunos added in AC an average damage model, not very realistic, people would've complained, so devs kept it as it was in the first place, basically textures. Maybe some day Ilja will add in AC some detachable meshes for cars and particles, so at least you will see some "shattering shards" on the track, that will be the compromise.