From what I can tell, Youtube "apps" on other platforms are simply wrappers around the website, not true native apps. Apple removed support for webviews from tvOS (here's a good explanation: https://medium.com/bpxl-craft/apple-tv-a-world-without-webkit-5c428a64a6dd). Apple TV Youtube app is developed separately and hasn't been updated for this reason. Youtube probably doesn't (yet) see a reason to devote a lot of resources toward developing a full-featured app for it.Thing is, the PS4 had far superior apps on day one, and they're updated regularly. I know the difference between a thriving, supported platform and one which languishes unloved.
If you have an explanation for the big YouTube update skipping the ATV, please do share. Something's fishy.
On the bright side, this looks promising: http://www.kendallinnovations.co.uk/#!tv-tube/s6294