Why cars rust

I've sprayed the inside of the doors, inside of fenders and over the wheel wells on my 90 Dodge truck every spring & fall since it was new. I am the original owner. It doesn't get driven much anymore, but it used to go out daily as it was my only transportation until 2006. It has over 200K miles on it right now and NO RUST anywhere.
When it was about 15 years old the power door lock motor needed to be replaced in the drivers door, and when I removed the door panel there was a layor of this thick grease looking stuff in the bottom of the door from all the coats of WD40 I had sprayed in through the drain holes. I still spray it down twice a year with the stuff. I use a whole can in the spring (after a full rinse with a garden hose) and a whole can in the fall. Works great, it's easy to apply and the stuff is cheap.
I was into old Vettes for years, and they have a big problem with chassis rot up here. I bought only cars with solid chassis', but whenever I bought another old Vette, I would coat the inside of the frame with Marvil Mystery Oil. It is thin enough to spray out of a syphon gun on an air compressor. Once that is done, you can drive it in the Atlantic Ocean without worry! LOL I used to park the car on the grass and spray it down. Insert the gun through the stamped holes in the frame and blast away. The MMO would atomize and get blown into all the small passages. You could see it drip out of holes 7 feet away from where the syphon gun was inserted. I just moved the gun around until I felt I had the whole thing coated well.
I recommend both of these practices to anyone who plans to keep a car for a long time. People saw my truck completely stripped of paint in my garage in 2007 when I was repainting it, and couldn't believe there was no rust, and you could see there was no bodywork done. There was no primer on it to "hide" anything. The truck was in a bare metal state in my garage from March until August.

George