Because the process of legislating is messy, bureaucratic, and complex.
And the president's job isn't?
No. The President generally doesn't have to make unpopular decisions if he doesn't want to.
Also, each individual American has never voted for the vast majority of members of Congress. It's much easier to disapprove of people you've never voted for. Notice that, while Congress has a whole always has a terrible approval rating, combined approval for individual Senators and Congressmen always averages to far higher than the overall rating.
