Well, a few reasons I can think of:

1. It introduces an additional set when cloning the repo, git doesn't clone the submodules by default.
2. It's messy, because we now have all kinds of stuff in the repo, rather than just the one file we actually use.
3. Submodules can cause all kinds of strange problems when switching branches. Best to limit those problems to when it's worthwhile.

How does it make it look better on github?