Marvel auditioned dozens of actors to play Thor, and ultimately settled on a guy who had recently moved to America and already landed major roles in three movies. I don't know what the rest of your paragraph has to do with casting.
Poitier is literally just a stunt guy, he had done stunts on all (I think? maybe almost all) of Whedon's shows. 2/3 of the roles he's played have numbers after them, I'd have to imagine the rest are background parts. No one would even know he "played" Thanos if it hadn't leaked from wherever it leaked; he's not credited, and not one person involved with the film has mentioned his name. They needed a guy to come in, put on some makeup, and smile, so Whedon brought in a stunt guy he was familiar with who probably worked for next to nothing. That is not casting an actor. Nor is what he was required to do in the film acting. A Gap commercial is a more demanding role. They aren't going to trust this individual to portray the most terrifying villain in the galaxy.
They can stick whatever actor they do end up hiring in prosthetics to make him look the same, or go with an all-CG Thanos from here on out. But they will hire an actual actor to play him.
