The tl;dr is: Control. Vendors want it, making it easy to run open software would make that control harder to keep.

We have plenty of open software.

What we need next is open hardware.

Hardware specifications are still mostly secret, and the lowest level code that interacts with the hardware is still mostly distributed as an unreadable binary file.

As long as our hardware is full of secrets, running our own software on it will be like exploring a cave without a light.

The hardware is generally not open source, and it’s technically illegal in a lot of jurisdictions to redistribute the binary blobs that make it work without permission from the manufacturer (basically never has happened). Because there is minimal focus on forward and backward compatibility, different chips from the same product line are very often incompatible with each other, let alone from different manufacturers.

So phone ROM maintainers have the job of stripping out those binary bits from a recovery image for a given phone or something and then getting them working with an open source build of android. All the while taking a (albeit kinda low) legal risk. Take a look at most of the long running open source Android projects and you’ll see they have had to structure their projects to mitigate legal exposure.

In Linux most of the hardware support is just straight up open source, the bits that aren’t (typically GPU or specialist hardware drivers) are manufactured by people that want you to use their hardware on Linux, so they release prepackaged drivers for you to use.

Phone manufacturers are motivated to try and ensure everyone uses their stock ROM because ultimately they have to support the device if a user breaks something, and that’s easier for them when they eliminate as many variables as they can. Even if they can charge the user out of warranty, it’s still tying up support resources from their perspective.

Hardware incompatibility between phones. The PC world is much better standardized.

There are 2-3 major roadblocks. Like others mentioned we simply lack drivers. But in my opinion the biggest issue is the lack of a BIOS equivalent for ARM systems.

Most PCs use x86 processors while most phones use ARM processors. And most x86 systems come with a BIOS (nowadays UEFI). That is a very simple operating system that runs when the computer starts and it gives a standard way for other operating systems like Windows and Linux to start up. That way the same x86 operating system can start on just about every x86 computer.

ARM lacks this standardisation. Although there do exist some projects to fix this they are not in use on most phones. So you actually have to make a custom way to just start up for each and every phone.

On top of that and the general lack of drivers many manufacturers lock their phones down so that only their own operating system may start.

What did Google say that you didn’t understand?

Mainly two reasons, one about architecture, and one about vendors

In the PC world, the entire ecosystem is designed to be modular, and people expect to be able to put windows/Linux on any pc and have it work despite the manufacturer. The kernel just wakes up on one of the cores, figures out the CPU, wakes the rest of the cores, and from there it figures out the rest of the computer. By contrast arm systems are tightly integrated, each SoC is unique and there’s no way to figure out the rest of the system, the kernel wakes up on one of the cores, reads out what SoC this is, and mostly has to already know the chip and any additional hardware connected to it.

But, sure, there are only so many SoCs (kinda), and displays, cameras, and touchscreens are mostly similar, you are bound to find a way to tell the kernel what hardware is running on and have it work, right? Except a lot of phone hardware is proprietary (duh) and requires bespoke proprietary drivers, google pretends to encourage vendors to submit their drivers upstream, but this doesn’t really happen. Now, if you are familiar with running external drivers on Linux, you probably know how picky the kernel is in what to load, but android’s kernel is specifically modified to be less picky, to allow vendors more slack. Mind you, the API is not more stable, the kernel is just less picky.

Bonus: running Linux on arm laptops is indeed proving kind of a challenge (nothing impossible, but resources are limited), that’s because they are built like a mobile phone.

Phones have very different hardware

Touch screen

Accelerometers

Compas

Gps

Various antennas etc

So theres a lot of stuff needed that i dont think is part of … the linux kernel.

That said, isnt android open source?

Just fork it, strip out the stuff you dont want, add the stuff you do want

Let’s clarify some terminology.

Android is an operating system, not hardware. Android uses the Linux kernel, but differs greatly from desktop-oriented Linux distributions. Most phones are designed for Android, a bit like most PCs are designed with Windows in mind.

Desktop-oriented Linux distributions have a semi-standardized software stack with Linux, GNU libraries and utilities, a shell, X11 or Wayland, some sort of window manager or desktop environment, etc…

Other comments have explained how the hardware makes it difficult to have generic operating systems that install easily on any phone like we do for PCs, but they do exist. Ubuntu Touch and PostmarketOS are examples of desktop-like Linux distributions for phone hardware. It’s possible to install and use these on certain phones, but there’s usually a feature or two without a working hardware driver. Desktop Linux on laptop computers used to be that way too, but far fewer laptops have missing drivers now than a decade or two ago.

I have PostmarketOS installed on an older phone. I don’t think the user experience is quite ready for most people to use as their primary phone, even for me, and I’ve been running Linux on laptops for most of my adult life.

LineageOS?

Because a PC tells the OS what hardware is present. Phones do not, that’s why there is no hardware detection.

Nobody forces phone makers to support the standard that would just do that: https://www.arm.com/architecture/system-architectures/systemready-compliance-program

Because that would make a terrible phone

Stock Android pretty much does, but it isn’t what you think it is..