Because they will likely buy another thing in that same time. You don't need an entertainment box immediately. You can wait, save, and buy an entertainment box that can do multiple things.

You might not be able to play the latest and greatest but you can still play many games and you don't lose access to them. They are shutting down the Xbox 360 store soon, thus they'll lose access to any games they don't have downloaded. I have games on Steam older than time itself that I can still download, even if the publisher has delisted them and stopped them from being sold. I know people who still use laptops from 2005 to play indie games. Essentially pretty soon Xbox 360s are going to turn into disc-only consoles where a 600-dollar computer would never revert to that and people today play on computers from 20 years ago. It's rare but it certainly happens, especially in the Linux crowd.

Lastly, you can always upgrade a computer part by part. Which doesn't require knowledge of how the hardware connects. Just take it to a shop and tell them you want it to run faster for a game and they usually will do some inspections, charge you 100 dollars in labor and then whatever for parts, and get your machine upgraded.