At least according to his own blog posts, he tried to work from within to change Amazon for the better, to the extent that was feasible without making his own life a living hell.

When things went too far for him, he resigned in protest. And yes, it is a privilege that he is able to do so and still live comfortably, but that doesn't make it a meaningless gesture.

Now that he's no longer in a position where saying what he really wants Amazon to be/do/have done to it will lead to those around him vilifying him on a daily basis, he's using his privilege, and his voice, which speaks louder than most in the tech world, to try to get that to happen.