It is IO bound, but it is also CPU and memory bound as well. I say this because most of the loading a game does is while it is running. This is especially true for large open world games (and others to varying degrees), where sections of the world (including geometry, textures, audio, music) are being loaded and unloaded all the time.

Given this is happening while a game is running, it means that the loading code has to compete for CPU and memory with all the rest that is going on in the soft realtime system that is a game.