Why do almost all phones have to be in that narrow band of 6.5 to 6.9 inches?

I wish there were more size choices on both ends of the spectrum. While most people prefer more choice below 6", I would like some choice above 7", since I keep my phone in my belly pouch, and never use it one-handed. My current Huawei Mate20X is actually ok at 7.2" (but worse than the Mediapad X1 I had before which at 7" was actually wider) but is way behind on Android updates, and will soon stop running my banking app.