Because they won't be on Youtube forever and it's nice to have them, especially if you can't get online.
Frankly I believe that YT (and probably the channels that uploaded Bright Falls) will outlive GOG. And as I said, you can just download vids from YT. You can use tools such as the Real Downloader or one of those websites that provide download links to those vids. But yeah, I admit: It wouldn't be too bad if the episodes were bundled with the GOG release.