Why can't I commit an empty folder in git?

The reason for this is actually simple - directories in the git index only exist as part of the file paths.

If you have directories dir1 and dir2, with dir1 containing fileA and fileB and dir2 containing fileC, git will commit the following (i.e. add the following to the index file):

• dir1/fileA
• dir1/fileB
• dir2/fileC

In other words, neither dir1 nor dir2 are truly committed, they are simply constructed implicitly based on the paths of files contained inside them.

Now whether there are any deeper reasons for this or it simply made the implementation easier, I don't know.

GIT FAQ is pretty straightforward about it:

Currently the design of the Git index (staging area) only permits files to be listed, and nobody competent enough to make the change to allow empty directories has cared enough about this situation to remedy it.

Link:

https://git.wiki.kernel.org/index.php/GitFaq#Can_I_add_empty_directories.3F

It's actually entirely possible to create and commit and external directory using the low-level commands git mktree and git commit-tree (git ls-tree is useful for inspection). I think this is useful for mirroring other VCS's that do support empty directories. It's only the high-level commands that don't support it.