Why each device needs a modified version of Android? topic

I was wondering why you usually need an specific Android build for pratically each device in the market. With Desktop OSes like Windows and Linux-based we have the convenience of being able to install them in any PC as long as they have the minimum requirements and you have all the drivers. With Android, you need to download an specific build for your device, and sometimes there are absurd situations like users who have a 4G or dual-chip version of certain device having to wait longer for an android update.

So what do you think is the reason we can't just download Android and install it in any device we wish? Drivers? Lack of some sort of comom BIOS/UEFI/Firmware? Lack of standards for mobile devices?