system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is there a technical (non-monetary) explanation for why a game console like the PS5 wouldn't be backwards compatible with all PS4 games?

Every year a new console launches, only supporting a handful of games from the previous generation. 

I always assumed this was for monetary exploitation, and to not demolish the sales of the previous console on the pre-owned market.

But I'm also interested in knowing if there's an actual technical limitation behind this decision.
gpt
Unlike PC games, console games can be really tightly integrated and optimized with the console hardware because the game authors know \*exactly\* what hardware they're going to run on. This is part of why a console can pull off more intensive games than a computer with equivalently powerful hardware.

But...this means that the game is written assuming all that hardware is available. The whole point of a new console is to give the developers new, more powerful, more capable hardware to write their games on. To make a PS4 game run on a PS5 you have to include an extra "layer" in the PS5 to translate for the PS4 game. The PS4 game doesn't know it's on a PS5 and it expects PS4 hardware; the PS5 needs to handle those requests and make the fact that it's a PS5 invisible to the PS4 game. This means, at bare minimum, a bunch of extra software to write & test. If there was a format change or specific hardware functionality that isn't used at all on the PS5, you might also have to install the extra hardware (and related software to run it) just to support the PS4 game. 

That's all doable but you have to do it as an explicit and intentional effort to run backwards compatible games, it can't just happen by accident.