> But I work at a large company where all the source code is in a single repostory.

Sure, Google does this with perforce.

You are correct, that if you desire a single truly massive repository, you will eventually have trouble with git. However:

> It's just not scalable to large organizations.

Of course it is. The solution is to have multiple repositories and then use something on top of git to tie the repositories together as needed. For example, this is how the android project is being managed:

http://source.android.com/download/using-repo