Lincoln is in a different category, but Steven Spielberg made a brilliant choice by avoiding a biopic. But it's only indirectly about slavery. There have in fact been dozens of films about slavery. Most of them apologetic films. The moonbeams and magnolia fantasies of white dominance. One key example is The Birth of a Nation, which shows how lascivious and rapacious and venal black people were. They had to be controlled by slavery; if not, they would rape white women. That was clearly a white racist fantasy.