The entire point of YouTube's feature to programatically mark a video as under Creative Commons is to allow programmatic use of that video in YouTube's video editor. From the YouTube page on Creative Commons (emphasis mine):
Creative Commons licenses provide a standard way for content creators to grant someone else permission to use their work. YouTube allows users to mark their videos with a Creative Commons CC BY license. These videos are then accessible to YouTube users for use, even commercially, in their own videos via the YouTube Video Editor.
You can license your videos however you want. If you want to license them under CC-BY 4.0, or CC-BY-SA 3.0, or anything else, you can do that; just state a license grant so in your video description. YouTube is simply giving you a box to tick that says, "Do you want to license this video under CC-BY 3.0 (possibly in addition to any other simultaneous license grants)?" If you tick that box, users can use your video in YouTube's video editor. You could also license your video under CC-BY 3.0 without ticking that box (e.g., by saying so in your video's description), and it would effectively be identical to ticking that box, except YouTube wouldn't programmatically know it was safe to use as input for the video editor tool.
This also explains why they don't work particularly hard to clarify what version of the license used is. YouTube wants you to create more video within YouTube. They want to make it easy and legally safe for you to use YouTube's video editor to make more videos on YouTube. The ability to take a CC-licensed YouTube video and use it outside of YouTube is an ability that certainly exists, but it is outside the business interests of YouTube to emphasize it.
As for why the chose CC-BY 3.0 specifically: CC-BY is the simplest Creative Commons license, and 3.0 may have been the latest available license at the time they rolled out the video editor feature (or at least 3.0 may have been the latest version that YouTube's legal team had vetted as safe to use).
As why it's not an "international version": it is an international version. Version 3.0 of the Creative Commons suite includes versions that are "ported" to particular jurisdictions, and an "unported" version of each license type, which is intended to be applicable to all jurisdictions. YouTube links to the CC-BY 3.0 unported license. See the "Further Internationalization" section of the CC 3.0 writeup.