Because the call doesn't support rolling back partial progress. Imagine if you interrupted `mount`, and it had written to the directory table but hadn't yet ini

Because the call doesn't support rolling back partial progress. Imagine if you interrupted `mount`, and it had written to the directory table but hadn't yet initialized the underlying filesystem and now the kernel reads uninitialized memory whenever you look at that directory (or, less disastrously, the mutex that protects that table is left locked and you can never mount or unmount anything ever again).