... when I was 6 or 7, and assumed the box was just like the packaging for any other toy, and therefore disposable (didn't matter so much anyway, as it was cassettes for an Amstrad CPC, and the case doesn't provide all that much protection).

Actually, I've thrown away cartridge packaging more recently. Was strapped for space at one stage, so I threw out my SNES boxes (not the instructions) and the outer-packaging for some Game Gear games. I'm pretty sure that I wouldn't want to sell them on anyway, and don't have room to have them on display, so why keep them?

CD/DVD games are a different matter - they serve a protective purpose.