How come people think natural selection is the same as chance?

It's not an either/or situation. Yes, evolution is to a degree predictable, but in other ways it is not. You might want to read some Stephen Jay Gould, who has written extensively on evolution. It isn't as though science is a matter of settled conclusions. If you examine some of the dead ends that fossil evidence shows and the natural history of the planet, you could not come away with the conclusion that the outcome of evolution was inevitable or was not dependent on some degree of chance.