Why does art history exist as a distinct discipline separate from the discipline of history?
Coming at this far after the OP, but as an art historian I felt compelled to reply.

The reasons for the Art History's separation from History are myriad (political, pedagogical, philosophical, etc. etc. etc.). In general, art history is rooted in objects/visual materials, and requires training in the methodologies and historiography of visual analysis. Many of the tools of art historical work overlap with those of historians––we are informed by broader methodological shifts (psychoanalytical theory, feminism, postcolonial debates, etc.) and have our own set of theoretical paradigms that have extended beyond the realm of art history (Bildwissenshaft, formalism, iconology, etc.). It's a vast field that in recent decades has splintered into related disciplines (visual culture, visual studies). Depending on his or her methodology, an art historian could likely be as well-versed in the "historical" literature as the "art historical" literature in any particular area. Art historians are, I believe, departmentally separate (or often groups with studio art programs) due to the emphasis on object-based research and teaching. That said, art historical research requires the same level of rigor and sophistication as any other form of historical research.

While many art historians choose to go the curatorial route, many who pursue advanced degrees stay in academia. Working in a cultural institution like an art museum requires a distinct set of skills from that of an academic. Some graduate programs in art history gear themselves more toward the curatorial route, while others are more focused on academic research.

One element of art history that can, I think, be a bit confusing to the outsider is the interpretative element. Any strong analysis will be rooted in historical "facts," but when examining a work of art, there's always going to be some measure of interpretation or speculation. Again, if a conclusion is not borne out by strong research, it will generally not be widely accepted. But the extent to which art historians think creatively about their objects of inquiry is one of the many reasons the field can, to me, be so invigorating.