Why does Android not include an Add Hardware Wizard? 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?
On desktops and laptops, drivers are a necessary evil. Hardware is often user-serviceable and prone to changes, and there is a huge variety of possible combinations. On phones and tablets, this isn't quite so. You can't really change the hardware easily. It's entirely safe to disallow driver installation since you don't really need it.
It is a lot easier to break ARM devices than x86 with messed-up drivers.
Drivers on Android must be written for specific kernel versions and all. This is easy to do for a manufacturer, but hard to get right as a user.
Most USB On-The-Go devices are USB interfaces, for which drivers are included. 
Driver installation requires root access, which is disabled by default on the vast majority of Android devices.
Allowing driver install exposes a massive surface for the installation of rootkits and other malware.  And, in fact, such malware can persist across devices wipes and firmware upgrades.


We will likely see some kind of driver installation if Project Ara takes off. But I imagine it will be along the lines of each component having its driver loaded onto it and installing when attached, as opposed to allowing the installation of arbitrary drivers found on the Internet.