cab corner on driver side rusted out. Chevy dealer says not covered if truck has 100k or is 10 tears old. I have seen other trucks same year and style with same problem. Why did some rust and others didn't?

My truck has severe rust bubbles and visible round rust spots above the rear wheel well openings. It's really the only 1999 I've seen with this ugly problem !

The spots started as a small pin hole, then just bubbled from underneath, no previous repairs and original paint.

Rust spots are starting to show, in one area, lower area. i had gotten and esto. they said the need to cut bad area out is going to cost me 1,000.00 dollars at Chevy dealer. also they said this is a common area that seems to rust on the trucks that he sees, I feel that GM should fix this on going problem. I did not no this was going on till i saw brown spot on outside paint lower area. does anyone out there have same problem?