Slight digression: why isn't a computer – a general purpose computer, open enough to run mainline Linux – in this form factor readily available? I'm fine with not calling it a phone. I just don't understand why we don't have (connected) open pocket computers by now, with all the innovations introduced by smartphones more or less commoditized by now.

By "open" above, I don't necessarily mean open hardware (though that would be great). I just mean "as open as a random consumer x86 computer you can just throw any Linux distro at without any special secret sauce".

https://www.gpd-minipc.com/ has all kinds of models for the "tiny laptop" form factor as long as 7 inches is tiny enough for you. Their recent products seem to be pivoting more towards the "handheld gaming computer" space, but their Pocket 3 and MicroPC 2 seem to be pretty close to "pocket computers". As far as I know, these are plain old amd64 platforms that will run Linux as well as any other Chinese motherboard.

The small+portable nature of these phones make them unsuitable for amd64 chips (so far) so everyone is using ARM chips, which means dealing with weird and quirky bootloaders or hard-coded OS keys. Qualcomm is putting effort into getting some iterations of their hardware into a well-supported state, so hopefully we may see better mainline Linux support on their chips soon. However, you're not going to get your hands on Qualcomm chips if you don't beat their (high) minimum order quantities and these tiny keyboard phones are hardly mainstream devices, so they often end up with MediaTek chips which have absolutely terrible mainline Linux support (and even worse bootloader quirks).

Miniaturization is expensive and often these kinds of devices rely on some form of subsidy to be cheap enough to reach popular adoption levels. Not to mention the user interfaces these days seem to be built for touch or game controller, and not a lot else.

Your options are things like the CHIP (which is dead, now, I think?), Pocket GPD or other gaming focused ultra-portable, or something like the Pinephone.

Linux phones exist, but I don't see a huge market for it. Most people don't really need a general-purpose computer in their pocket. The most used app on my phone is a web browser, and I also need a banking app, authorization apps and all the parking, public transportation apps for wherever I happen to be today to work.