People keep weird stuff in their source repositories. Over the years, I observed mobile development frameworks stored in Mercurial, SQL Server installation media stored in Perforce, network captures stored in SVN, large test datasets stored in pretty much every source repository type, and most certainly a lot of 3rd-party source stored alongside with the project source.
The common belief is that a source repository should contain everything that is needed to build, package and test project binaries and these processes should not rely on external sources. This sentiment does make sense on its own, but keeping binaries and test data in source repositories simply turns these repositories into oversized junkyards, storing everything, but the kitchen sink.