There’s a single and obvious reason. Bigger screen.

And if you’re going to ask why a bigger screen? It’s the same reason your laptop or desktop isn’t 7” wide.

My laptop isn’t 7” wide because I do my work on it, and that work involves reading long text files and requires a keyboard big enough to comfortably type said long text files, not because “bigger screen = always better”. It’s all about the use case.