Clamshell folding phones also have external displays.
certainly, but here's what apple has to say becomes relevant: space issues. the display has thickness, even if you fuse it together with the touchscreen. it also generates heat - not a lot - but enough to mandate some sort of thermal management: whether that's an air gap or some thermally conductive material -> adds to thickness. not much, but small things tend to accumulate. then you want to have it shock/scratch/break-resistant too.
then there's the UI complexity and the problem of screen real estate: another weirdly shaped screen has to be handled either by the OS or by the installed apps. how much stuff can you fit onto it, how can one interact with it. not impossible, but pretty challenging as the various screen sizes will certainly pose some challenges to app developers, so there might be some apps that won't support the external screens and require you to open the phone anyways.
or the OEM might just decide to restrict the use of the external screen solely to the OS to provide an uniform experience.
then there's the problem of taking and making calls with the device - I know this is probably not the main function of nowadays' smart-"phone" - but still it needs to be dealt with. so what are the options:
- speakerphone when in clamshell mode (works, but has the obvious issues. the AW also works as a speakerphone, but that's more like some emergency function)
- some sort of headset/earpiece (probably the most feasible way, but requires extra items that need to be sold/charged/cared for)
- open the damn thing whenever it rings
texting: sure, you can do the old fashioned yell-at-the-phone speech to text way, or have some quirky letter by letter input or a tiny unusable virtual keyboard too - just like the AW. is it working? kinda yes, but I wouldn't want to type this post on such a thing. again, you need to open the thing up.
and this basically boils down to keeping opening/closing the device whenever it comes to any social interaction with it.