The skin is distributed as an XML file for a few reasons. Since the template edits are not in just one file it can not be distributed as a single skin file. There are custom macros and template bit edits to the post section as well as the template bit group.

By using an imported skin XML file it ensures that skin inheritence works correctly and you can continue to edit your skins from within your ACP.

Simply uploading the skin_*.php file is not sufficient to correctly, completely install the skin. If your XML skin import function is not working properly you should search out a remedy to fix that problem before continuing with installing.