OP isn't claiming that evolution is directed. He's simply illustrating the magnification effect of existing bodily structure on intelligence mutations.
OK I think I've just ended up reading OP in a such a way that it appeared OP was saying that primates evolved opposable thumbs because they needed opposable thumbs, as if evolving opposable thumbs suddenly resulted in primates thinking "you know what would be awesome? If we could touch the pads on our hands together."
There isn't a "because".