I won't spoil the reason, but I have to say it's answers like the one given that make me feel like GNU coreutils is essentially decades of hacks thrown together haphazardly in a bunch of loosely related and begrudgingly coordinating pieces of software. I enjoy these code spelunking articles though!
One might be wondering why 'test' and '[' are separate binaries at all. This is to give maximum flexibility wrt copying binaries etc. Note one can build coreutils like busybox as a single binary, by just `./configure --enable-single-binary` at build time. This is already available as an option on Fedora at least through the coreutils-single (1.2MB) package.