I gave a talk at Game On Expo about decompiling Castlevania: Symphony of the Night (https://github.com/xeeynamo/sotn-decomp ) earlier this year and talked a little bit about exactly this. Almost everyone who works on loves the game. After that, motivation varies: some want to see ports, some want to mod, some want to learn everything they can, some want to preserve. Along with those I also like the challenge (not unlike sudoku).

Doing it long enough requires learning compiler history and theory, understanding business and engineering pressures of making the game, and occasionally reveals why parts of the game work the way they do.

I stream working on SotN and am happy to answer any questions in chat if you’re interested in learning more - https://m.twitch.tv/madeupofwires/home