B) The Windows filesystem (and to an extent the POSIX) isn't just "local", it also includes transparent and semi-transparent network file storage.

C) Windows and POSIX are both multi-user and multi-process. They operate over multiple cores and multiple I/O buses.

Even if it just one system API centralized in charge of all that, it still needs to be built on top of complex distributed dance of mutexes/locks/semaphores/other distributed control structures. Because of the nature of I/O control there are complex caches involved and transaction semantics of when data is actually pulled from/flushed to low level data stores. The transaction model in turn is reflected in what the files look like to other users or processes running at the same time.

Windows and NTFS combined have strong transaction guarantees that other users and processes must see a highly consistent view of the same files. It makes heavy uses of locks by default even for cached data. POSIX favors the "inode" approach that favors high availability and fewer locks at the cost of eventual consistency and the occasional partition (the "same" file can and will sometimes have multiple "inodes" between different processes/users, many common Linux tools rely heavily on that).

Those two different transaction models are most easily explained in analogy to CAP Theorem. They are very different transaction models with different trade-offs. Whether or not you see a single "local machine" or you see a distributed system of cores, processes, users, diverse I/O buses is partly a matter of perspective and will reflect how well you personally think CAP Theorem is a "perfect" analogy for a filesystem.