I have two main reasons.
1. The games themselves are not based off of Conway's game of life. They commonly are complex without being elegant. They are often poorly balanced, low in complexity within each siloed off gaming area (like skills, quests, combat, etc.), and require a great deal of repetitive behavior without enabling their users to script through it.
2. They limit players to what is programmed within the game's code. In a tabletop RPG the players don't know this code either, but the elements of their brilliant ideas not already encoded are simply programmed in on the spot. It's a yes, no, irrelevant so yes methodology rather than a yes, no, irrelevant so no. There is still a scope for knowing what is the game and what is not, but it is otherwise limited by players' linguistic ability rather than their computers' computing power.