The Apple Watch runs WatchOS. It's a stripped down version of iOS, optimized for a much smaller screen and much smaller battery. It also has its own, distinctive look and feel, including its own take on the Home screen, its own interaction methods like the digital crown and force touch, and its own, distinctive capabilities like the Taptic engine, optimized for a wearable.
In contrast, when Apple decided to launch the iPad, they used iPhone OS. It was originally suggested they use the exact same single-column interface, but because it wouldn't make efficient use of the much larger screen, they decided to implement a dual-column interface instead. Yet the Home screen and design language remained the same as the iPhone, as did the interaction methods and capabilities.
Imagine instead, like the Apple Watch, the iPad ran its own distinct version of iOS: iPad OS. Rather than a stripped down version for smaller screens and batteries, imagine it ran an amped-up version that really took advantage of bigger screens and batteries, with a Home screen, interaction methods, and capabilities optimized for a tablet.