A directory is a file like anything else that contains a map of names to inodes. If you're trying to add or remove mappings (create or delete files), then clearly some synchronization must occur or the contents of the file will contain garbage. In theory you could get away with a very small critical section that says "lock bytes N through M" of the file, but then how do you deal with disk block alignment (i.e. two pairs of n-m bytes are on the same disk block, so they need to take turns anyway) and how do you deal with I/O errors (the first n-m bytes fail, but the second n-m succeed, now you have a hole with garbage).

Also no need to theorize: run the benchmark I linked for yourself. It clearly shows a massive advantage to having each thread work with its own directory.