My favorite theory is an extension of #1: interstellar travel is so expensive and difficult that advances in AI and virtual reality make it undesirable. There's

My favorite theory is an extension of #1: interstellar travel is so expensive and difficult that advances in AI and virtual reality make it undesirable. There's no point in spending a non-trivial fraction of your civilization's resources colonizing another solar system when you can do anything you want at arbitrary time scales in a simulation.