Is Linux really a monolithic kernel? I mean yeah for most use cases such as desktops and servers it is but there's nothing preventing you from disabling most of its features and drivers and keeping a very minimal core (I'm sure lots of embedded Linux does this) and you can, right now, make it act as a microkernel by putting any component in userspace, mounting filesystems and devices via FUSE and networks via TUN, at its current state.

Now the question becomes why these components are built in the kernel and not in userspace? The answer is clear for each individual component. See how the in-tree NTFS driver eventually replaces NTFS-3g. Basically when an in-tree solution exists it just gets preferred over userspace solutions because of performance and reliability etc.