Part of it as already mentioned is a safety thing. Crumple zones and the like are there to purposefully deform so that the people inside the vehicle have a higher chance of surviving a crash.

Part of it is that being hit in the wrong way can also weaken the structural integrity of the frame making it unsafe to use. Makes more sense to strip it for parts at that point. Last thing a repair or insurance company wants is to be found liable for saying “yes the car is repairable/safe to drive”, then the front falls of on a highway.

Part of it also is that insurance companies won’t want to pay for repairs that amount to more than the cost of replacing the entire car if it’s older. Or they know they can make more money by paying out a policy then repairing and refurbishing the vehicle.