Because weighted boots are weightless in space. The planets all orbit the Sun at different speeds along orbits that are different circumferences, so they are usually distributed all around the Sun, as in this picture.
But in the mid 1970s, there was an alignment that made it possible for a single spacecraft to pass near multiple outer Solar System planets, as in this picture.
Remember the scene where Matthew McConaughey and Michael Caine are walking around that giant concrete building and Michael Caine says "Turn your bloody head sideways, Master Bruce." And Matthew McConaughey does, getting a new perspective on the orientation of features of the building and then says "Alright, alright, alright. This is a spaceship!"