The vast majority of games are very complex GUIs because the entire game is a graphics interface with the user, not just some HUD or menus. Yet most of them run

The vast majority of games are very complex GUIs because the entire game is a graphics interface with the user, not just some HUD or menus. Yet most of them run on an immediate mode API. It's funny that you mention this because I remember trying to utilize DirectX RM (retained mode) for a game in late 90s. Even without taking the performance into account (one could say that the implementation was not very optimal) it was a major pain to do anything. One thing I remember vividly: our explosions were static meshes containing all frames of animation and were animated through toggling visibility of different nodes. Luckily that game had been killed by publisher and I did not have to suffer through optimizing its performance.