In your case, having a screen resolution larger than the 3000 pixel width limit for header graphics, you will, unfortunately, see them like that, except where a design might not go across the whole 3000 pixel width and the remaining background color matches the background color selected when uploading a design.
All three designs that you show print screens of go across the whole 3000 pixel width of the image but their background colors are set to white. If the background color had been set to a more complimentary color (a matching color to what is at the far left edge) it wouldn’t be so bad. It would blend with the background color. It’s completely up to the designer though. They may not even think to take into consideration larger screen widths.
However, I do believe there has been some discussion about making the width of themes larger, at some point.