Because in a sense it's almost insulting. It's like saying you really didn't do a lot of research. I don't do my research anymore. Bernie does my research, but those early books were also well researched and I'm very proud of the work I put into them. The research certainly shows, but historians do research yet don't write in ways that touch you so.
Historians have nothing to do with fiction.
Historians, I think, are concerned with facts. They're not concerned with motive. Or character, really. They're not interested in why a person might do a certain thing, they just say that this event took place. That this particular artifact was produced by so and so at such and such an age.