Because it's another file to manage. The beauty of having all the main resources in a single template file is that it's very easy to manage.
At the moment if a change is needed to the template all I need to do is open the template, alter it, overwrite, and then propagate via Remote Desktop.
It means when someone asks for a change I can usually do it then and there. Adding anymore steps to this process (such as having to keep a duplicate non-template file in sync), I've found, makes it too time-consuming, easy to forget to do things, and less likely the change will get done there and then when it's most convenient for everyone.
It's also a case of simplicity. If you can tell people there's one file and one file only where they can get everything they need then it makes life a lot simpler.