This is a good observation. You can have a feature list for a game and complete every one (hah, who am I kidding!), but if the end product isn't fun, so what? It's not going to be successful, and all your effort is wasted.

There's the in-principle estimatable work to implement all of the game systems and then there's all the other unpredictable work to actually make the damn thing fun.

Feature-creep is endemic because creative directors/stake-holders can just move the goalposts or insist on their pet features / vision.