Because I went and forgot to get my Ph.D. in history, smartass.
You got one? You write 'em.
A 30 second google search on "Salmon Chase book" gives over a million hits. Right at the top is this collected works of his papers and scholarly work on his life. The books are out there.
And why does it take a PhD to write a definitive work? For an example, Shelby Foote had no PhD, as near as I can tell. Carl Sandburg didn't either.