It is indeed a builtin, but `/bin/[` still exists for compatibility reasons!

   $ which [
   /bin/[
   $ type [
   [ is a shell builtin

   $ which test
   /bin/test
   $ type test
   test is a shell builtin

Right, that's also a good reminder that the builtin `command -V` is typically what you want whenever you use not-always-builtin `which` ;)