If I want to install Windows on, say, a bare-metal Thinkpad X1 Carbon with no OS, then I can buy Windows and install it. Windows includes numerous drivers. If there are any necessary drivers missing, I can normally get them from the manufacturer, use the Add Hardware Wizard, and install them.
But, if I want to install Android 6.0 "Marshmallow" on, say, a Samsung Galaxy S Relay 4G, then things are not so simple. It's true that I could (theoretically) download and install some random Nexus ROM shipped by Google. But such a ROM includes far fewer drivers, and in fact may not allow my phone to boot up at all. And there's no UI for downloading and installing signed drivers downloaded from hardware-manufacturer websites.
Why does Android not include any driver-installation wizard?
And how does Android deal with a USB On-The-Go ("USB OTG") device, if it doesn't know how to communicate with that particular USB OTG device?