Can you not achieve the log history on a subtree with `git log my/subfolder/`? Tools like TortoiseGit let you right click on a folder and view the log of changes to it.

Yes it can, but the point is that in a git repo you store the entire history locally, so whenever you clone a repo, you clone its history on at least one branch.

So when you have a repo that's hundreds of GB in size, the entire history can be massive.

You can indeed. The problem with this strategy is that now you need to maintain the list of directories that needs to be checked out to build each project. And unless this is automated somehow, the documentation will gradually diverge from reality.