Directories and hardlinks take up space, just very little.
It would make sense to hardlink a directory if everything in that tree was unchanged, but no filesystem will allow hardlinking a directory due to the risk of creating a loop (hardlinking to a parent directory), so directories are always created new and all files in the tree get their own hardlink.
Apple's Time Machine was given an exception in their filesystem to allow it, since they have control over it and can ensure no such loops are created. So it doesn't have that penalty creating hardlinks for every single individual file every time.
It would make sense to hardlink a directory if everything in that tree was unchanged, but no filesystem will allow hardlinking a directory due to the risk of creating a loop (hardlinking to a parent directory), so directories are always created new and all files in the tree get their own hardlink.
Apple's Time Machine was given an exception in their filesystem to allow it, since they have control over it and can ensure no such loops are created. So it doesn't have that penalty creating hardlinks for every single individual file every time.