Google, like many tech companies, has a habit of imposing its design language wherever it goes. Amazon does this, too, which is why its Prime Video app also looks and behaves the same on the Apple TV as it does on the PlayStation 4 and elsewhere.
Often times, having the same design language across multiple platforms is a good user experience. If you know how to use an app on just one device, you'll know how to use it on anywhere.
The problem is, not all devices are built the same. Some computers, like iPhones and Android phones, rely on touch controls; other computers, like PCs, rely on keyboard and mouse inputs; other computers, like streaming devices, rely on voice controls for the best experience.
When it comes to the YouTube app on Apple TV, it's about time Google and YouTube embraced the Apple TV for what it is: an atypical streaming device with some nifty features that make life easier for users, like inertial scrolling and voice control.
Google and YouTube employ some of the best software engineers in the world, and they've done an excellent job at making Material Design consistent; now it just needs to be flexible.