to add to what @HughesTooling and @TrippyLighting said, I'll offer one other reason why region detection is limited to a single sketch: To allow you to segregate regions by sketch.
A sketch is really just a container of curves and points. This allows you to create groupings that make sense to you. If you want all those lines to participate in regions, why would you not want them in the same sketch?
The performance point that Peter makes is a good one. If Fusion had to look around at other sketches to do region detection it could get quite expensive, and lead to failures. Consider this case.
Start with a simple sketch:

Extrude the results, and create one sketch on one face:

Create another on the other face:

If Fusion supported this kind of region detection, the overlap of those circles would be a valid sketch region:

And you could, for example, Extrude it. But, if you go back and edit the original sketch so that those faces are no longer coplanar:

then Fusion would have to figure that out, by looking at each sketch in the entire design, and reject those that are not coplanar. It would be quite expensive:

Anyway, this is a good discussion, and I want to encourage more of these kinds of discussions.
Jeff
Jeff Strater
Engineering Director