It depends what you mean by "it". FUSE clearly doesn't give you every feature in plan9, and in fact you can't have that without giving up the current Linux syscall API completely and replacing it with something vastly simpler that leaves a lot more to be done in user space. That's not something that Linux is going to do by default, seeing as they have a backward compatibility guarantee for existing software. Which is totally OK as far as it goes; the two systems just have different underlying goals.
Plan 9 supports file server processes natively, and that's the part that's most FUSE-like. The full OS also has many other worthwhile features that are not really addressed by FUSE on its own, or even by Linux taken as a whole.
One key difference is that the equivalent to kernel syscalls on *nix generally involves userland-provided services, and this applies to a lot more than just ordinary file access. The local equivalents to arbitrary "containerization/namespacing" and "sandboxing" are just natively available and inherent to how the system works. You can't do this out of the box on *nix where every syscall directly involves kernel facilities, so the kernel must have special provisions to containerize, sandbox, delegate specific things to userland services etc.
