Biologically/evolutionary, why would humans prefer junk food over health food? Why do we resist exercise when?

exercise is so obviously beneficial? Is this just a cruel joke being played by God? What I mean is, from an evolutionary standpoint, I would think that we would be biologically programmed to crave healthy foods and exercise because it results in a longer life. Instead it's just the opposite and we have to force ourselves to stay away from too much fat/sugar/salt, and we have to force ourselves to exercise. Is this some sort of test we have to go through to prove that we can resist temptation?