ELI5 backwards compatibility

Or rather backwards incompatibility. With the Switch 2 being officially announced I became curious about how a game system could not have backwards compatibility. I don't really understand computers or how a game system works but to me they are basically just computers that run on their own OS. My understanding of a new console is that they basically just add a better processor and up the graphics or whatever and put it out, so why would a game developed for the previous system not work on a newer system?