Android tablets fall under the "not about... electronic devices, media players, cell phones or smart phones" clause in our help center. The operating system is essentially locked down and inaccessible without specific tools, with the tool used highly dependent on the device.
They are a computer in the same way a smart-fridge is a computer. They might have a processor that computes things but they are far from being "general purpose" computing devices with hardware and software that is user accessible.
Outside of the relevant store having an app to do what you want they lack any real low level access.
The hardware is entirely custom and generally only the manufacturer knows the exact details, and the same goes for the operating system.
"Computers" on the other hand have a reasonably fixed platform architecture, and can predictably have access to USB devices, PCIe, mostly have interchangeable processors and memory and are at their heart general purpose devices. Don't like how your computer works? Wipe it and install an OS that does what you want inside an hour. The instructions necessary for one computer will almost certainly work on any other computer.
Tablets are difficult to repurpose due to their locked down nature. They have processors but they are far from being general purpose computing devices. You cannot simply turn it in to an elite gaming PC or a headless Linux box without throwing it away and buying something more appropriate. If you want to repurpose the hardware then generally you need to have first spent a week researching that one device to find out what others have picked apart from it. The instructions used to modify one device will almost certainly turn another into a brick without that research.
If you consider a tablet to be a "computer" then we should probably also consider a $2 pocket calculator a computer as well, after all it does compute the answer to "2+2". The line has to be drawn somewhere, and we drew it between what are essentially glorified media players and "real" computers.
The specification of the word "computer" might well be vague, but the exclusions are clear enough in my opinion. General electronics with a processor in it does not count as a "computer". At best tablets are glorified big screen smart phones which are explicitly off topic per our help center on topic page except insofar as they interface with your computer
See this answer to a discussion on what we wanted to allow.
We have other sites for more specific mobile devices such as Android Enthusiasts and Ask Different.
To address why some tablets are on topic: because some tablets are real computers.
There are "tablets" that are based on standard computer hardware and use a "full" desktop operating system such as Windows or Linux. Those tablets are considered on topic because on the software side anything you can do on a normal computer is relevant to those tablets. The hardware might be non-replaceable but is still the same hardware you would find in a desktop or laptop. Android and iOS tablets are custom hardware and software for every device and there is very little cross applicability.