A modern car body is engineered to destroy itself in an accident. That is not a failure of strength but the mechanism by which occupants survive impacts that would otherwise be fatal.
Deceleration, not speed, causes injury
Injury results from how rapidly a body changes velocity. Stopping from a given speed over a longer distance reduces the forces involved proportionally.
A rigid car would stop almost instantly against a solid object, transferring the entire change in velocity to the occupants in a very short time.
Deformable structure buys distance and therefore time, and every extra centimetre of controlled crush reduces the peak force the occupants experience.
The structure is graded from soft to strong
Front and rear sections use materials and geometries designed to fold progressively, with crush initiators built in so the collapse begins where engineers intended.
Behind them the passenger cell uses high-strength steels and closed sections to resist deformation, preserving survival space and keeping doors operable.
The transition between the two is the critical design region, since energy must be absorbed before the load reaches the cell and the cell must not collapse afterwards.
Load paths spread force around the occupants
Impact forces are directed along defined routes through the sills, floor cross members, roof rails and firewall, distributing the load across the structure.
Multiple parallel paths mean no single member has to carry everything, which is why modern cars have several longitudinal rails at different heights.
Offset and small overlap impacts are the difficult case, because only part of the structure engages and the load has to be redirected across the vehicle.
Compatibility between vehicles is a related problem. A tall vehicle's main rails can sit above those of a lower one, so the structures pass each other rather than engaging, which is why beams at a common height have become part of the design brief.
The interior restraint system has to match
Crumple zones control the car's deceleration, and belts and airbags control the occupant's, which is a separate problem happening milliseconds later.
Pretensioners remove belt slack and load limiters allow controlled webbing extension, so the chest is decelerated over distance rather than stopped abruptly.
Airbags manage the head and provide a deforming surface, and their timing is calculated from the deceleration signature the structure produces.
Why repairability changed as a result
Structures designed to deform in a specific way cannot be straightened arbitrarily, since heating or pulling high-strength steel alters the properties it depends on.
Manufacturers therefore specify sectioning points and replacement components, and repairs outside those procedures compromise performance in a subsequent impact.
This is a substantial reason why apparently moderate damage results in a vehicle being written off, and why repair method matters as much as repair cost.