I'm not entirely sure what you mean by this. Operating systems don't typically include drivers out of the box for every single interface that could possibly connect to it. Often you'll get 'generic' drivers on Windows that at least map some of the basic inputs, but up until like late Windows 8 iirc Windows didn't even include that. Previously if you wanted to connect ANY controller to your PC you had to install third party drivers to make that work. So Valve bundling their controller drivers with steam just kinda... makes sense? Are you saying you would prefer to go find the drivers or have them written by not Valve instead? I really don't understand the 'walled garden' take here. You could go build your own drivers for this if you really wanted to, you don't need to use Valve's software.

USB HID protocol has been designed with huge flexibility and self descriptive devices in mind so that in theory you shouldn't require a custom drivers for vast majority of current input devices even controllers order of magnitude more complex than anything currently sells. Just like you don't need a custom driver for each usb flash driver.

In practice half-assed HID drivers by OS, badly designed OS<->application APIs, hardware manufacturers copy pasting HID descriptors from other devices, not following the standard properly, firmware bugs getting fixed with drivers instead of firmware fixes, intentional discrepancies from standard, console manufacturers reinventing the wheel has lead to the current mess.

But they're not drivers included with steam. In order for it to work you must launch the game through steam.