Why isn't "Landscape Mode" fully exploited?

I really lust for the iPhone 7 Plus just because it has landscape homescreen, but I got fooled once with iPhone 6 Plus thinking "wow a landscape iPhone all use this ALL THE TIME" until I realized apple had no plan to make iOS fully landscape, the way we know it on the iPad. Some apps are still portrait only, the keyboard is still awful, even Apples own apps aren't all usuable sideways and I really don't understand why? I would really use it sideways all the time if it was optimized like it is on the iPad (never used iPad in portrait)

What do you think?