Why are all full-size tablets now designed primarily for landscape use?

Not talking about the iPad of course.

All new Windows 8.1 tablets and Android tablets, especially in sizes >8" diagonal, seem to be primarily designed for landscape use with 16:9 and 16:10 aspect ratios. A tablet needs to have a 4:3 ratio IMHO. It feels more natural to hold a tablet in portrait mode like a magazine. Web-browsing looks comical when you hold these tablets vertically. Wide screen mode is good when you are trying to type something, but for most other use cases, portrait seems to makes more sense. I feel holding a wide aspect ratio tablet in landscape mode introduces a bigger cantilever, there by making even a light tablet feel heavy in the hands.

What do you think tablet manufacturers are doing this? It just seems unergonomic. What am I missing here, or better yet, why doesn't Apple think this is a good idea? Is this just another way competitors are trying to differentiate themselves from the iPad?