Android is an operating system, in the linux family.
You can download android source code all you want - like other open source projects.
You can run android on your laptop.

Software needs to be written and compiled for a specific processor (and to some extent, platform).
Your desktop or laptop is always going to be based around an Intel x86 or x64 platform, or the AMD chips.
Windows is made to work with those, no matter who manufactures the box.

Your phone and tablet don't have chips like that. They aren't designed to load a full version of Windows. They don't have a bunch of replaceable/upgradable RAM, and you can't swap out the GPU.

There have been versions of Windows made for mobile.

It isn't about Android. It is about the phone builders. You can force the booting of your phone, crack the loader, and load any software you want on your phone - if it is compatible with that hardware. If you have enough storage and RAM.

You don't want to load Android 8.0 on a flip phone. You can't load the Apple OS on a Moto Droid.
The software and hardware need to be compatible. They aren't developed in a vacuum.