It's only about whether the game runs on SteamOS, handhelds and screen size, input and performance thereof don't matter for the rating like it does for Deck Verified.

It already show if controller are fully/partially/not supported

And they also show which controllers are supported or if you need the steam input to translate the commands

Steaminput means they all work, even when they don't.

You can make pretty much anything control the mouse.

Touchscreen is about same

That’s a great point! Maybe they expect a consumer to connect their own mouse if their handheld doesn’t have touch screen or trackpads.

Some like it, others don't. It's more important that consumers have options rather than absolutely everyone being forced to agree on one way being best.

this philosophy applies to many things, not just controller layout.

Valve and Sony are the only ones that did symmetrical. Symmetrical was actually first with the DualShock for the PS1, but the 360 controller being popular and commonly used for PC apparently made googly sticks more common. The Switch at least has a justification in that the joycons can be removed and used by themselves, necessitating the stick placement.