Symlinks are specific to individual devices; also, symlink implementations are different from operating system to operating system.

So, if Google Drive or Dropbox were to support that, they would have to implement some very complex solutions (along with some probably very confusing interaction with the user) in order to be able to sync reliably across devices, even devices running the same operating system.

You can always[1] move your Documents inside the Google Drive folder, and then put a symlink at the place Documents were.

[1] I haven’t tried this with Google Drive yet but I suppose it should work. It is what I do with Dropbox, in Linux and in Windows. It works fine.