Well that might be true for some people when reading paragraphs of text. But even for text I personally prefer reading wikipedia on fullscreen, not the mobile layout. That's mostly because all the boxes take up even more horizontal space. If they used the margins for infoboxes or to bring footnotes closer to the section referencing them like gwern's site does that would be a different story.
But the bulk of API documentation isn't continuous text it's quite structured and could benefit from a more grid-like, columnar layout that's used when there's enough screen estate. And I don't mean newspaper-style multi-column layout, I mean aligning different kinds of information horizontally.
See, there's not just the red space wasted. If we're specifically talking about where-clauses then it's also wasting the green space.
Compare to java
The current layout may be fine for people with small windows on a single screen. But it doesn't work well when you bring up documentation fullscreen on a separate monitor.
especially when it needs to support the narrow layout anyway.
Well, different layouts can be applied based on window width. The javadoc table linked above is not actually a table, it's a grid that folds in the last column on smaller widths
