Why cars rust

when i worked for a shop in cali not more than a year ago... we had a mid 00's car come in with a problem
everytime they would fill it it would smell like gas...

so i lifted it up and the entire bottom side was like swiss cheese... control arms had holes in them...

the filler neck rusted away and was leaking on the ground...

looked at the plate and it was from NY...

we just had a donor car come in for one of our SS Hemi Darts, it was from maryland or something, i had to do both .25's, trunk and both x-tensions, bottom half of the rear window lip area, both front floor pans, and more in the engine bay...

F**k back-East cars... just crazyness