Old stuff gets relegated to the garage, basement, or attic for alot of people. These places are not well climate controlled, most of the time. Dust piles on then gets wet, games get dumped in boxes with stuff they shouldn't be stored with, containers get spilled on there. Then there are just accidents, spilling a soda on the coffee table where the games are stacked up or whatever. After all, I have an NES cart that was leaking... something. So I know what you mean.
Oh yeah, and americans treat their belongings like shit in general. There's always that.
Atari carts are the ones that get me. The labels, sheesh! They all look like they were soaked in grease.