Also, keep in mind that hardware complexity has increased exponentially since the 80s. That you have the specs doesn't mean that you can easily implement a driver. Have a look at the free driver for Radeon videocards, for example. Despite having the specs and massive support from AMD, after years in development the driver is still not up to par with the proprietary driver, in terms of performance and functionality.

Maybe we should look at something within the last two decades on the way computer hardware and their drivers work... ya know, something closer to when the Linux kernel was actually created.

Is it the way I think it should be? No, but that doesn't change the fact that hardware is hidden by closed source drivers. Some are able to figure out how to interact with the hardware without utilizing closed source binaries, but it isn't common, and it most certainly does not occur on the majority of phones and tablets. That is why all of them require binary extraction from the original ROM before you can build your own version (pre-built custom ROMs like CM already include these binary files). But a lot of work needs to go into the OS to interact with those binary files. That's why there are separate versions of ROMs for each device. That is why you can't take Slackware and install it onto a tablet. It would need a lot of work to tailor it to that specific device, and it would only work on that device. This is why ROMs like CyanogenMod have individual device maintainers and tons of other people working on it (this is all the people who submitted work to gerrit up to April 2014). Good luck trying to get enough people to do something like that for Slackware (I certainly have no desire to run Slackware on my phone). Then you'll run into the same thing that every blog post on CM's site runs into with people asking for support for their random device. Providing phone/tablet support is a completely different beast than providing it for regular computers/laptops. I'd say you can't honestly expect them to be the same, but it seems you already do.

I think you're trying to compare dissimilar things. Platforms are different than hardware. The various ATX platforms are very open. You have specs for the various connections and slots. Examples include how you can know what the individual pins do on a serial connector, usb plug, PCIe slot, or HDMI cable. If you have the know-how, "you can create hardware and write your own drivers for said hardware". But the hardware you buy that plugs into those are usually not opened up. This has been the case for a very long time and doesn't look like it will change any time soon. Plus the more complex something gets, the more the company feels it needs to keep its secrets guarded. Hardware back then was not very complex compared to today's hardware.

If Slackware don't, someone will figure it out