Is it a rule imposed by the phone carriers, who want you to buy a different plan with no voice service if you have a tablet? It can't be the phone makers since the are so many. It can't be Android software licenses since Apple seems affected too. I'd be pretty interested in a tablet sized phone. But they seem to have maxed out in the current tall skinny format that is not really big enough for some things. Just wondering.
Edit: aha, I managed to get rid of the stupid photo. Thanks for the help.
Basically, ~7cm width is about the limit it seems, because I assume manufacturers realised that's where most human hands are comfortable with holding a phone.
There were a few giant phones in the past, Sony Xperia's Ultra at 6.4 and the Xiaomi's Mi Max at 6.44" to 6.9". If given the 19:9 to 20:9 aspect ratios of today they'd be 7" phones (so again, don't compare phone sizes based solely on screen diagonal). As they're no longer made I assume they just didn't sell well enough.
The phone that I use right now (Moto G Stylus) is about 15cm wide when I use it in landscape mode. It is perfectly usable that way, except that it is nowhere near tall enough.
Giant phones like 6.9" are not from the past, they are also in the present, like the "pro" versions of the Pixel and Iphone. Why are they almost 7" instead of actually reaching 7"? That's what I'm trying to understand. I'm very skeptical of the "they didn't sell well enough" theory, which sounds to me like system justification. It also doesn't explain why I can't buy a phone sim and put it into a tablet. So absent concrete evidence, it still sounds to me like carriers are getting in the way of larger phones. I do see though, that the Samsung Z Fold3 foldable has 7.6" diagonal when unfolded, so maybe that refutes the carrier theory.
I think people might be less concerned these days than before about pocketing or handholding the phone, since they use connected wearables like the Apple or Google watch for small screen functions like phone calls.
This is my assumption, the demand for people that want a 7" phone does not justify the cost of development and manufacturing. Yes, there will be people that would want it, but just like the iPhone Mini, the demand wasn't justified to continue producing the smaller phone.
Generally 6" to 6.5" seems to be the sweet spot that is large enough to use, and small enough that it fits in my pocket comfortably.
My guess is people weren't buying large phones anymore. Those that want them are getting foldables. That being said you could get the Galaxy Tab A7, I think there is a version with a 4g modem. Also there are iPad mini models with 5g iirc.