"Mental illness is the same as physical illness"
You can't use "social constructionist" views of one kind of illness and not the other.
In other words, why not a "social constructionist" view of cancer? The answer is that it simply makes no sense to take this view of cancer.
The fundamental problem for "mainstream psychiatry as practiced today" (MPAPT) is that there are no physical tests for any of the psychiatric "disorders" (with certain irrelevant exceptions).