[rdl]

They do this; for front (especially) and rear impact, you have the entire crumple zone, with lots of cross-hatched struts and other components to take away energy in deformation.

There are various side impact technologies to try to 1) prevent ultimate penetration and 2) absorb energy. The problem is the doors need to be relatively thin, relatively light, and contain a bunch of other components, as well as function as doors. Multiple layers is one.

The best right now seems to be use the door to prevent penetration, and use a side curtain airbag to reduce peak acceleration on passenger (along with seat design and seat belts).

I personally picked used cars no earlier than ABS and side curtain airbags whenever friends asked for cheap used car options.