Another aspect to this is that Android is Linux, but it is not GNU / Linux. This is true both in the literal sense of not using GNU coreutils or glibc, and also in the broader sense.
What I mean by the "broader" sense:
no X or Wayland
GTK or Qt support is something an application has to bring with them.
filesystem is substantially reorganized
users and system permissions setup substantially differently
To the application programmer Android / Linux looks like a completely different ball game.
Its like saying MAC is UNIX, technically yes but not really
macOS is UNIX-based. Period. In fact, “[s]tarting with Leopard, macOS has been certified as compatible with the Single UNIX Specification version 3 (SUSv3).” Source
As far as I can tell literally everything (except for windows) is UNIX, it isn’t exactly a high bar to pass, it’s like saying all drinks are water based which is technically true but I’m not going to give a kid a wine box when I run out of juice boxes.
I think that Linux is synonymous with open source and freedom and to some extent privacy.
android is synonymous with Google
I’m only guessing, but when people ask about a Linux tablet or phone, they actually want the former, rather than the later, even though android is a Linux derivative, as you rightly said.
There's been multiple attempts Ubuntu and Plasma both have projects, Ultimately a lot of compatibility issues seem to hamper uptake, components like cameras and such can be hit or miss.
More than anything, the problem is apps. I installed Ubuntu Touch on an old device I had lying around (after replacing the battery to bring it back to life), and I ended up liking it more than my daily driver. It worked, the interface was snappy (no pun intended), and there's a lot of solid design choices. I found myself trying to navigate with gestures on my android phone after.
However, I could not talk to my friends, who unfortunately use WhatsApp. I could not install my banking app. There's a bunch of small thorns in the side that makes changing difficult for most users - kind of like in the early days of desktop Linux.
As in the early days of desktop Linux, the solution might lie in a compatibility layer (like WINE). Android emulation within at least Ubuntu Touch has gotten quite good, I believe. Unfortunately my device is too old to support it.
Those interested should also check out Postmarket OS.
GNOME, a major desktop environment, is also moving towards all apps being designed for all types of devices. But as long as people can't use their banks, switching is hard.
True, its a real chicken and egg situation. No one what to use a platform without decent app support but no one wants to develop for a platform without any users.
There was also the Nokia N900 which ran Maemo, which was much closer to a traditional Linux distribution to the point that it was fairly trivial to compile desktop Linux programs for it. Which could even be done on the phone itself, if you had a lot of time on your hands.
The processors and peripherals aren't as uniform as they are on computers. Each SoC has a different set of components, which need drivers and custom set-up mechanics and they're often only shared between the chip designer and phone manufacturers. A lot of complicated reverse engineering is needed. And that's often not worth it as there are many different phones on the market and they're short-lived devices. So we end up in the current situation where we don't have the operating system foundation / drivers available to the community to build upon.
There is Linux on phones. A modified Linux kernel runs on every Android device. Just the userland is an entirely different one. And Linux isn't there yet with the desktop and apps. A lot of features are missing, power saving, delivering messages while sleeping and doing background tasks while saving power in the pocket... It's kind of a chicken and egg dilemma. We'd need some popular Linux phone so people are motivated to tackle the remaining software issues for a mobile ecosystem. And we won't get one unless there is some good software so it can be marketed.
Nothing prevents you drom installing a linux os on an android phone, provided someone made a rom for your device. It's just not very useable due to proprietary drivers. Ubuntu touch exists for example, but on most phones you can't even use the cellular antenna iirc
Nothing really. Someone else already wrote that Android is technically Linux. But you can also get "regular" Linux on mobile devices, it's just a matter of using a distribution with mobile-friendly UI as well as getting driver support for the phone's hardware. Ubuntu touch exists for example, but only works on a limited number of devices and even on supported devices it's not really suitable for the average user yet.
This is the key - no universal (i.e. No open standard) BIOS like on PCs.
We need a BIOS war for ARM, and since Android has it locked down, there's little competition to push for it, unlike when DOS was trying to become the standard for minicomputers.
In practice? Constantly changing hardware with soc vendors that publish nothing and device manufacturers that (have to) keep pushing out new models on a short cycle. Plus many of them have extra shenanigans to keep the bootloader locked so you can't install a recovery (presuming you had a working one) so you could replace the os. There are some rare exceptions, but the hardware is rare and tedious and not many people can or will work on installable Linux on them.
If you just want to run some Linux userland, there's ways to do that on top of android, though. Want to get to a Linux like system or run a program? Might be as close as installing a terminal or running adb shell.