Factor uses ahead of time compilation. When you load a source file at the REPL, all definitions in it are compiled immediately, and compiled code is saved in the image. So when you download a pre-built binary package, all the code has already been compiled so auto-detection is not an option.

There are pros and cons to both compiling at load time or run time; one of the disadvantages of the former is less flexibility when it comes to using CPU-specific features.