You said output is useless

No I said it's not very useful.

There’s zero cost to using it instead of a div so the only reason not to use it is to purposefully screw users who need accessibility features.

No, the main reason is because people don't know about it, because it's an extra thing to remember for little benefit. Same reason other semantic-only tags like article get very little use.

only differ from div in styles

Err yeah the "only" do a really useful thing that lots of people want.

If output came with some nice styling and maybe animations... maybe a built in copy-to-clipboard option... then people would have used it.