Up to roughly the end of the 19th century (and in some cases even in the early 1900's) an architect was also a structural engineer, and a structural engineer also an architect (and BTW in the classical times the architect was also the structural engineer and the site director, in some cases even the builder/contractor).
The gap between the two professions has become - in my experience - wider, and with the (needed) addition of other professional and "specialized" figures (geologists, plant engineers, etc.) each profession has become narrower in scope, to the point that sometimes it is difficult to have an architect and a structural engineer communicate properly, as they each lack a "global" vision on a project.
... and viceversa.
As said above as I see it the real issue is the (modern) separation/specialization of those professions.