This might seem like a shallow choice, much like choosing his long-time collaborator John Williams for best composer, but it's also the right one.
Spielberg's directing versatility is unmatched. How many directors can say they have directed films covering the science fiction, family, drama, monster, adventure, and war genres? Okay, maybe B-movie director extraordinaire Roger Corman, but he certainly can't say they were good, at least not in the same non-campy way Spielberg's are.
Don't get me wrong, there are some Spielberg directorial efforts I'm not a fan of. Not even mind-erasing (see Eternal Sunshine of the Spotless Mind) can make me forget the mess Indiana Jones 4 was. But overall his track record is second to none, with 11 Oscar nominations for Best Director and Best Picture, winning both awards for Schindler's List and Best Director for Saving Private Ryan.
Stanley Kubrick and Danny Boyle are two other directors with comparable versatility and consistency who come to mind, but neither can match Spielberg's level of output, the latter because of his young age, the former because he is mostly dead.
And, while I don't like to use box office numbers because even Michael Bay could be defended, a lifetime U.S. gross of $3.7 billion has to mean something. I have nothing but respect for directors who make a career out of appealing to a niche audience, but in my opinion just as much talent, if not more, is required to direct films that have enchanted mass audiences from the '70s to the '00s.