There are technical reasons that make such a feature hard to
implement. In particular, picture zones are defined in original
image coordinates, in order to be unaffected by things like rotation
and dewarping. That works fine for polygons, however it would be a
problem for rectangles / ellipses, as those won't be rectangles /
ellipses anymore after modifying image rotation / dewarping grid.
Additionally, no one is working on Scan Tailor at the moment.
However, some forks of Scan Tailor actually have the rectangular
picture zones feature. Check "Scan Tailor Featured" out.