Why do videogames have such a short shelf life?

[Retrogamer0001]

I'm not sure if anyone has brought this point up already, but I think that many companies (besides Nintendo) are catching on to this very trend - after all, just look at all the classic games available on the PSN and VC, and with the Xbox Arcade. Many of these classics are available to everyone with an internet connection and 5-10$ in their pocket, and it doesn't cost any retailer self space.