The reason PCs are still open is historical, they started like this and people can't really see PCs any other way but they're a dying breed and whatever comes after is a different game with different rules. We'll have some niche manufacturer building something more open for the small segment of enthusiasts.
The fact that so many people casually say how a platform is open or flexible because if you jump through enough hoops you get to make some minor modifications to your device, or applauding the openness of a system for allowing you to sideload standard apps is the reason manufacturers don't see a need to propagate the PC philosophy anymore. Too many users grew up with this new model so they don't see the loss.
None of the devices you mention are user serviceable in any way hardware-wise, the software is almost always locked onto the device and there's no easy, or even supported way of changing it. And if you do change it you just get a small variation of the same software. Your phone may get a slightly degooglified version of Android but it's unlikely you'll get it to run Ubuntu, Windows, or a FreeBSD without a massive investment in time and knowledge.
I don't expect or even really want to swap hardware on these devices: if that was a priority for me I would get a PC, Arduino, Raspberry Pi, etc. I get the desire to tinker and customize, but from where I'm sitting I have plenty of opportunity to do that.
I also don't expect the device manufacturers to do the work to make Ubuntu or FreeBSD run. They didn't do that for PCs, the community did.
> We'll have some niche manufacturer building something more open for the small segment of enthusiasts.
I would argue that's exactly what Raspberry Pi and Arduino is.
This is why you need rpi specific distro builds.