Because ARM is a design licensed to companies that want to make their own chips for their own purposes. Those purposes involve optimizing cost by choosing, per product, whether to integrate various cores onto the SoC, or to leave them as external devices on the board, or to maybe put them into a secondary chipset microcontroller and route some IO pins to connect the two.
In ARM designs, the licensee determines the required pin-out, because the licensee determines what the CPU does vs. what the board does. You can't really have a generic "ARM board", because no two "ARM CPUs" would have the same expectations for what are on that board.
A particular ARM licensee could standardize the socket between their own designs—but in doing so, they'd lose a lot of the advantage of licensing vs. buying an off-the-shelf chip in the first place.
https://www.youtube.com/explainingcomputers is a good YouTube channel for reviewing these types of single board computers.
There's Qseven [1] and SMARC [2].
1 https://en.m.wikipedia.org/wiki/Qseven
2 https://en.m.wikipedia.org/wiki/Smart_Mobility_Architecture
We have reached the point where shipping 1.2B Smartphone per year is called massive, comparing to 200M PC market where 150M of those are laptop ( Soldered and non Socketed ).
You cant buy a self made ARM PC. But there will be Qualcomm ARM Laptop. And the AWS ARM CPU are based on ARM N1 Design, it is only a matter of time somebody make the same CPU, or System integrator selling CPU made from other ARM CPU vendor such as Fujisu, Ampere, Huawei, Marvell, or possibly Nvidia and others.
I dont see much of a problem in getting ARM access as consumers.
Google, which has previously collaborated (OP1) with Rockchip on Chrome OS.
It's not hard to acquire something with an ARM CPU on it, but at prices an ordinary person can afford, they're all in the category of toy computers. Try to find an affordable ARM system with a M.2 2280 NVME SSD slot on it like you can find on a $100 desktop x86-64 motherboard. Or multiple PCI-Express 3.0 x8/x16 slots.
I've previously spent many years working for a hardware manufacturer. Personal theory is that this is a real example of a chicken or egg problem related to economies of scale. Nobody wants to spend dozens of millions of dollars tooling up to produce ARM socketed CPUs and motherboards and stuff, which may or may not be price/performance competitive with current-gen Intel and AMD stuff by the time it's ready for release. And there's a huge risk in manufacturing something like that and then discovering that the sales volumes are really low.
Look at the sheer massive quantities that the top-ten Taiwanese motherboard manufacturers churn out every year.
> You can wipe pretty much any Android device (not just phones, but also HDMI "streaming boxes", which are a convenient form-factor for basing a workstation on) and install Linux on them
No you really can't, and phones aren't servers. Take any modern $600 smartphone and try installing something very close to a stock debian or centos on it. Being able to maybe boot a Linux kernel on something doesn't mean that there's anything like the market demand for that particular hardware platform target for a whole distribution.