As the name (Open Broadcaster Software) suggests, it isn't primarily a capture program.

> even if that means capturing too much

People streaming things mights disagree.

Except it's really designed to composite video from a bunch of different places and broadcast the result, whether they are windows, screens, cameras, etc. And a rather lot of people don't want every screen to be captured, just the ones that they have selected. It's not a "screen recorder" program, really. And it's explicitly designed with an API to be remote controlled if hardware changes out from under it, so you can encode exactly your own policy logic on what to do. (External display plugged in? Disable my laptop source at full resolution and enable my laptop and monitor sources, side by side, with one scaled as necessary, and in this layout. But if I'm currently recording or streaming, don't do anything. - this is user-specific logic)