FreeBSD has Linuxlator because there is a lot of binary only software that was never and never will be ported to BSD, so it's necessary for them in order to avoid bleeding users away. Conversely, macOS has basically all software ported natively to it, so when you _need_ a Linux environment 95% of the time it isn't because you need $XYZ that only run Linux, but because you need a proper Linux environment with systemd, cgroups etc. Implementing that stuff on top of XNU would probably be extremely expensive and it would arguably defeat the point of having their own kernel in the first place.