Because you are under a misapprehension. It is simply not true. Capture requires a third body to alter the potential energy (gravitational versus kinetic), changing its path from hyperbolic to elliptical. Bodies captured our solar system are captured only when they have a close encounter a planet or other orbiting mass. If they do not, they will fly right on through and out the other side, continuing on their hyperbolic trajectory. Same with ejection. (Which is identical to capture, but time-reversed). No body can be ejected from the SS without having an encounter with one of the sun's children. It needs to alter its elliptical path to hyperbolic in order to escape.